This will help mitigating any discrepancies between the issuer
configured and the one returned by the OIDC provider.
This also removes the need for configuring the `account_management_url`
explicitely, as it will now be loaded from the OIDC discovery, as per
MSC2965.
Because we may now fetch stuff for the .well-known/matrix/client
endpoint, this also transforms the client well-known resource to be
asynchronous.
