Feature: Single Sign-On through OIDC/OAuth2

Continuation of !80 (closed).

The goal of this MR is unrelated to the implementation of MSC3861 (the delegation of authentication via OIDC), although this would still benefit the implementation of the former proposal by incorporating the authentication protocol. This MR merely aims at allowing login through m.login.sso, dropping the requirement of a password during registration.

TODOs

• extend the configuration with relevant options
• attempt to also support for OAuth2 providers like Github and Facebook
• allow OIDC Discovery in place of specifying each endpoint manually
• implement fallback pages for user interaction requirements
• UIA support
• allow multiple identity providers per account
• properly test all identity providers supported by Synapse
• provide backchannel logout
• custom claims support
• allow complete or partial migration of existing accounts
• configurable attribute boundaries (not sure why Synapse supports this)

Some of the above goals were features requested by other users/contributors and this functionality is not necessarily part of any specification.

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• I ran cargo fmt and cargo test
• I agree to release my code and all other changes of this MR under the Apache-2.0 license