Allow upgrading existing synapse users to upstream oidc providers on import

[matrixbot]

This issue was originally created by @MTRNord at matrix-org/matrix-authentication-service#3135.

It would be great to say on import or afterwards that a certain user should be "upgraded" to match a certain upstream provider user. Currently, I am guessing this would be rather hacky to achieve by manipulating the db which is neither supported nor sensible.