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

[sandhose]

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

[hughns]

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

[pmaier1]

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

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#2088