Support OIDC tokens signed with HMAC-based algorithms

This issue has been migrated from [#13391](https://github.com/matrix-org/synapse/issues/13391).

---

HS256, HS384 etc are symmetric signing algorithms for JWT tokens (see [RFC7518](https://datatracker.ietf.org/doc/html/rfc7518#section-3.1), [Auth0 docs](https://auth0.com/blog/rs256-vs-hs256-whats-the-difference/)).

In this case the signing key cannot be retrieved via a JWKS URI, as we would for an asymmetric key (since the key is private), so it must be statically configured.

To support this we would need:
* A way to configure the key for symmetric signing algorithms
* A way to configure the `id_token_signing_alg_values_supported` (or just extend the default).

See also https://openid.net/specs/openid-connect-core-1_0.html#IDTokenValidation.