Allow User Login Admin API to return refresh tokens

[anoadragon453]

Imagine I have configured Synapse (legacy) SSO with nonrefreshable_access_token_lifetime: 24h in order to encourage admins and users to use refresh tokens.

I also have an admin tool that I have an account for, and would like to log into that account using the User Login Admin API.

Currently, I can do so and receive an access_token. But this access token will expire in 24hrs! I'd love to be able to get back a refresh token that I can use to refresh the access_token before it expires every 24hrs.

Proposal

Similar to the User Registration Admin API, we add a refresh_token body parameter to the User Login Admin API. This would then cause a refresh token to be returned in a refresh_token field in the response.

Admin tooling could then refresh the access token in order to get back a new access token with a fresh 24hr lifetime.