Normally, when `discovery` is enabled,
`id_token_signing_alg_values_supported` comes from the OpenID Discovery
Document (`/.well-known/openid-configuration`). If nothing was
specified, we default to supporting `RS256` in the downstream usage.
This PR just adds support for adding a default/overriding the the
discovered value [just like we do for other things like the
`token_endpoint`](1525a3b4d4/docs/usage/configuration/config_documentation.md (oidc_providers)),
etc.
