Implement refresh tokens

[langleyd]

Add support for refresh token:
https://github.com/matrix-org/matrix-doc/blob/main/proposals/2918-refreshtokens.md
matrix-org/synapse#11427

Implementation Highlights:

• We add the refresh_token parameter to login/register requests to receive a refresh_token(used to request new access tokens) and an access_token in addition to a expires_in_ms interval (to estimate client side when the token expires).
• We should handle the cases where refresh_token is empty (server may not support refresh tokens or not have it enabled), expires_in_ms is empty ( the access token does not expire).
• There is a /refresh endpoint where the refresh_token can be provided to gain a new access token.
• Refresh tokens rotate, each time we call the refresh endpoint we also get a new refresh_token. Therefore we must be careful around the time of each refresh that if we have multiple concurrent requests we don't make multiple refresh requests or mishandle the new refresh_token.
• We can preemptively refresh the token shortly before it expires to minimise any disruption to the user(waiting on the refresh request before a user request can be made). On iOS the server timeout we set on sync requests is 30s, so sometime like 60s seems reasonable for preemptive expiry.

Rollout plan:

1. Distribute an alpha build or maybe add to dev feature flags.
2. Enable for all beta users and monitor for a period of time.
3. Ship to prod and monitor.