Allow fetching user claims through the userinfo_endpoint on upstream OAuth 2.0 logins

[matrixbot]

This issue was originally created by @sandhose at matrix-org/matrix-authentication-service#2088.

We currently extract user claims through the ID token. In some cases, we would like to fetch instead the userinfo endpoint to get those infos, especially on non-OIDC providers

Part of #2080

[matrixbot]

This comment was originally posted by @hughns at matrix-org/matrix-authentication-service#2088 (comment).

Using https://matrix-org.github.io/synapse/latest/openid.html as the basis, the following rely on the Synapse userinfo_endpoint setting:

• Gitea
• Twitter
• GitHub
• Mastodon

[matrixbot]

This comment was originally posted by @pmaier1 at matrix-org/matrix-authentication-service#2088 (comment).

We decided that this isn't required for the next milestones as we're not focusing on the OPs mentioned in matrix-org/matrix-authentication-service#2088 (comment).