Allows overriding the `redirect_uri` parameter sent to both the
authorization and token endpoints of the IdP. Typically this parameter
is hardcoded to `<public_baseurl>/_synapse/client/oidc/callback`.

Yet it can be useful in certain contexts to allow a different callback
URL. For instance, if you would like to intercept the authorization code
returned from the IdP and do something with it, before eventually
calling Synapse's OIDC callback URL yourself.

This change enables enterprise use cases but does not change the default
behaviour.

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Best reviewed commit-by-commit.

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Co-authored-by: Eric Eastwood <erice@element.io>