Allow for multiple social-sign-on for a single account

[deanveloper]

Your use case

What would you like to do?

I would like to link both my google and github accounts to my matrix user.

Why would you like to do it?

So that I can use either one.

How would you like to achieve it?

In the settings, there should be an option to add social sign-on options to an already-existing account.

Have you considered any alternatives?

There isn't a clear alternative to this, other than being forced to only use one account. I use my Github account for now, but would like to transition to using my Google account since the 2FA for Google is quite a bit simpler than Github.

Additional context

With matrix-org/synapse#9110, it looks like this should be possible from a protocol standpoint. But I am not entirely sure.

[t3chguy]

SSO is entirely done by the server

Closing in favour of matrix-org/synapse#9441