Adapt OAuth2 implementation to Matrix Spec v1.18 (#34026)

* Adapt OAuth2 implementation to Matrix Spec v1.18

* Handle more cases of oidc->oauth

* Fix test

* Fix read back of oauth2 context

* Iterate

* Fix tests

* Discard changes to apps/web/playwright/e2e/settings/account-user-settings-tab.spec.ts

* Fix test

* Fix test

* Fix test

* Potential fix for pull request finding 'Unused variable, import, function or class'

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* Iterate

* Iterate

* Fix test

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docs/oauth.md

@@ -0,0 +1,23 @@
+# OAuth2 and delegated authentication
+
+See https://areweoidcyet.com/client-implementation-guide/ for implementation details.
+
+Element Web uses [/auth_metadata](https://spec.matrix.org/v1.18/client-server-api/#get_matrixclientv1auth_metadata) to discover the configured provider.
+Where a valid configuration is discovered, OAuth2 native login flow will be the only login option offered.
+Element Web will attempt to [dynamically register](https://spec.matrix.org/v1.18/client-server-api/#client-registration) with the configured OP.
+Then, authentication will be completed [as described here](https://areweoidcyet.com/client-implementation-guide/).
+
+#### Statically configured OAuth2 clients
+
+Clients that are already registered with the OP can configure their `client_id` in `config.json`.
+Where static configuration exists for the OP dynamic client registration will not be attempted.
+
+```json
+{
+    "oidc_static_clients": {
+        "https://dummyoidcprovider.com/": {
+            "client_id": "abc123"
+        }
+    }
+}
+```