Add MSISDN validation API on the IS

This API has existed for quite a while, but historically we've instead proxied
this request via the homeserver. As part of MSC2290 work, we are changing
approaches such that we will request tokens directly from the IS when binding
for discovery.

Part of https://github.com/vector-im/riot-web/issues/10839

src/base-apis.js

@@ -1803,10 +1803,11 @@ MatrixBaseApis.prototype.registerWithIdentityServer = function(hsOpenIdToken) {
 };
 
 /**
- * Requests an email verification token directly from an Identity Server.
+ * Requests an email verification token directly from an identity server.
  *
- * Note that the Homeserver offers APIs to proxy this API for specific
+ * This API is used as part of binding an email for discovery on an identity
- * situations, allowing for better feedback to the user.
+ * server. The validation data that results should be passed to the
+ * `bindThreePid` method to complete the binding process.
  *
  * @param {string} email The email address to request a token for
  * @param {string} clientSecret A secret binary string generated by the client.
@@ -1818,12 +1819,12 @@ MatrixBaseApis.prototype.registerWithIdentityServer = function(hsOpenIdToken) {
  * @param {string} nextLink Optional If specified, the client will be redirected
  *                 to this link after validation.
  * @param {module:client.callback} callback Optional.
- * @param {string} identityAccessToken The `access_token` field of the Identity
+ * @param {string} identityAccessToken The `access_token` field of the identity
- * Server `/account/register` response (see {@link registerWithIdentityServer}).
+ * server `/account/register` response (see {@link registerWithIdentityServer}).
  *
  * @return {module:client.Promise} Resolves: TODO
  * @return {module:http-api.MatrixError} Rejects: with an error response.
- * @throws Error if no Identity Server is set
+ * @throws Error if no identity server is set
  */
 MatrixBaseApis.prototype.requestEmailToken = async function(
     email,
@@ -1864,6 +1865,74 @@ MatrixBaseApis.prototype.requestEmailToken = async function(
     }
 };
 
+/**
+ * Requests a MSISDN verification token directly from an identity server.
+ *
+ * This API is used as part of binding a MSISDN for discovery on an identity
+ * server. The validation data that results should be passed to the
+ * `bindThreePid` method to complete the binding process.
+ *
+ * @param {string} phoneCountry The ISO 3166-1 alpha-2 code for the country in
+ *                 which phoneNumber should be parsed relative to.
+ * @param {string} phoneNumber The phone number, in national or international
+ *                 format
+ * @param {string} clientSecret A secret binary string generated by the client.
+ *                 It is recommended this be around 16 ASCII characters.
+ * @param {number} sendAttempt If an identity server sees a duplicate request
+ *                 with the same sendAttempt, it will not send another SMS.
+ *                 To request another SMS to be sent, use a larger value for
+ *                 the sendAttempt param as was used in the previous request.
+ * @param {string} nextLink Optional If specified, the client will be redirected
+ *                 to this link after validation.
+ * @param {module:client.callback} callback Optional.
+ * @param {string} identityAccessToken The `access_token` field of the Identity
+ * Server `/account/register` response (see {@link registerWithIdentityServer}).
+ *
+ * @return {module:client.Promise} Resolves: TODO
+ * @return {module:http-api.MatrixError} Rejects: with an error response.
+ * @throws Error if no identity server is set
+ */
+MatrixBaseApis.prototype.requestMsisdnToken = async function(
+    phoneCountry,
+    phoneNumber,
+    clientSecret,
+    sendAttempt,
+    nextLink,
+    callback,
+    identityAccessToken,
+) {
+    const params = {
+        client_secret: clientSecret,
+        country: phoneCountry,
+        phone_number: phoneNumber,
+        send_attempt: sendAttempt,
+        next_link: nextLink,
+    };
+
+    try {
+        const response = await this._http.idServerRequest(
+            undefined, "POST", "/validate/msisdn/requestToken",
+            params, httpApi.PREFIX_IDENTITY_V2, identityAccessToken,
+        );
+        // TODO: Fold callback into above call once v1 path below is removed
+        if (callback) callback(null, response);
+        return response;
+    } catch (err) {
+        if (err.cors === "rejected" || err.httpStatus === 404) {
+            // Fall back to deprecated v1 API for now
+            // TODO: Remove this path once v2 is only supported version
+            // See https://github.com/vector-im/riot-web/issues/10443
+            logger.warn("IS doesn't support v2, falling back to deprecated v1");
+            return await this._http.idServerRequest(
+                callback, "POST", "/validate/msisdn/requestToken",
+                params, httpApi.PREFIX_IDENTITY_V1,
+            );
+        }
+        if (callback) callback(err);
+        throw err;
+    }
+};
+
 /**
  * Submits an MSISDN token to the identity server
  *