Pull out a public CryptoApi (#3211)

... so that we don't have to implement annoying shims in `MatrixClient`

src/crypto-api.ts

@@ -0,0 +1,62 @@
+/*
+Copyright 2023 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import type { IMegolmSessionData } from "./@types/crypto";
+import { Room } from "./models/room";
+
+/**
+ * Public interface to the cryptography parts of the js-sdk
+ *
+ * @remarks Currently, this is a work-in-progress. In time, more methods will be added here.
+ */
+export interface CryptoApi {
+    /**
+     * Global override for whether the client should ever send encrypted
+     * messages to unverified devices. This provides the default for rooms which
+     * do not specify a value.
+     *
+     * If true, all unverified devices will be blacklisted by default
+     */
+    globalBlacklistUnverifiedDevices: boolean;
+
+    /**
+     * Checks if the user has previously published cross-signing keys
+     *
+     * This means downloading the devicelist for the user and checking if the list includes
+     * the cross-signing pseudo-device.
+     *
+     * @returns true if the user has previously published cross-signing keys
+     */
+    userHasCrossSigningKeys(): Promise<boolean>;
+
+    /**
+     * Perform any background tasks that can be done before a message is ready to
+     * send, in order to speed up sending of the message.
+     *
+     * @param room - the room the event is in
+     */
+    prepareToEncrypt(room: Room): void;
+
+    /**
+     * Get a list containing all of the room keys
+     *
+     * This should be encrypted before returning it to the user.
+     *
+     * @returns a promise which resolves to a list of
+     *    session export objects
+     */
+    exportRoomKeys(): Promise<IMegolmSessionData[]>;
+}