As MSC2263 states, m.require_identity_server must be set to false when it does not require an identity server to be provided by the client for the purposes of email registration or password reset.
Adds an m.require_identity_server flag to /versionss unstable_flags section. This will advertise that Synapse no longer needs id_server as a parameter.
This is a combination of a few different PRs, finally all being merged into `develop`:
* #5875
* #5876
* #5868 (This one added the `/versions` flag but the flag itself was actually [backed out](891afb57cb (diff-e591d42d30690ffb79f63bb726200891)) in #5969. What's left is just giving /versions access to the config file, which could be useful in the future)
* #5835
* #5969
* #5940
Clients should not actually use the new registration functionality until https://github.com/matrix-org/synapse/pull/5972 is merged.
UPGRADE.rst, changelog entries and config file changes should all be reviewed closely before this PR is merged.
