Add support for overriding id_token_signing_alg_values_supported for an OpenID identity provider

[MadLittleMods]

Add support for overriding id_token_signing_alg_values_supported for an OpenID identity provider.

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, etc.

Todo

• Maybe add tests

Dev notes

poetry run trial tests.handlers.test_oidc.OidcHandlerTestCase

Pull Request Checklist

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[anoadragon453]

Seems fine to me. Tests to make sure that the value is overridden when the config is set, and not overridden when the config isn't set, would be appreciated.

I haven't looked at existing tests, but don't feel the need to build out discovery testing infrastructure for just this PR if it doesn't exist 🙂

[MadLittleMods]

Thanks for the review @anoadragon453 🦝