Create mas-oidc-client crate

Methods to interact as an RP with an OIDC OP.

crates/oidc-client/src/requests/client_credentials.rs

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+// Copyright 2022 Kévin Commaille.
+//
+// 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.
+
+//! Requests for the [Client Credentials flow].
+//!
+//! [Client Credentials flow]: https://www.rfc-editor.org/rfc/rfc6749#section-4.4
+
+use chrono::{DateTime, Utc};
+use oauth2_types::{
+    requests::{AccessTokenRequest, AccessTokenResponse, ClientCredentialsGrant},
+    scope::Scope,
+};
+use rand::Rng;
+use url::Url;
+
+use crate::{
+    error::TokenRequestError, http_service::HttpService, requests::token::request_access_token,
+    types::client_credentials::ClientCredentials,
+};
+
+/// Exchange an authorization code for an access token.
+///
+/// This should be used as the first step for logging in, and to request a
+/// token with a new scope.
+///
+/// # Arguments
+///
+/// * `http_service` - The service to use for making HTTP requests.
+///
+/// * `client_credentials` - The credentials obtained when registering the
+///   client.
+///
+/// * `token_endpoint` - The URL of the issuer's Token endpoint.
+///
+/// * `scope` - The scope to authorize.
+///
+/// * `now` - The current time.
+///
+/// * `rng` - A random number generator.
+///
+/// # Errors
+///
+/// Returns an error if the request fails or the response is invalid.
+#[tracing::instrument(skip_all, fields(token_endpoint))]
+pub async fn access_token_with_client_credentials(
+    http_service: &HttpService,
+    client_credentials: ClientCredentials,
+    token_endpoint: &Url,
+    scope: Option<Scope>,
+    now: DateTime<Utc>,
+    rng: &mut impl Rng,
+) -> Result<AccessTokenResponse, TokenRequestError> {
+    tracing::debug!("Requesting access token with client credentials...");
+
+    request_access_token(
+        http_service,
+        client_credentials,
+        token_endpoint,
+        AccessTokenRequest::ClientCredentials(ClientCredentialsGrant { scope }),
+        now,
+        rng,
+    )
+    .await
+}