This allows clients to specify the identity provider they'd like to log
in with for SSO when they have multiple upstream IdPs associated with
their account.
Previously, Synapse would just pick one arbitrarily. But this was
undesirable as you may want to use a different one at that point in
time. When logging in, the user is able to choose when IdP they use -
during UIA (which uses fallback auth mechanism) they should be able to
do the same.
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