Allow to link existing user:password to new OAuth login

Description:

Currently the existing accounts username:password are used to login via OAuth session (MAS).

If the user has assigned an email for the account, when trying to use this email to login the session still asks for a username (trying to create an account).

If the user has assigned an email, the user should be able to login via the email as if the user has created the account using the email in the first place.

Created this in Synapse [18382](https://github.com/element-hq/matrix-authentication-service/issues/4827) but thought more relevant to MAS