Support for id_token_signing_alg_values_supported

[timoschinkel]

It is good practice to specify what JWT signature algorithms are allowed: https://curity.io/resources/learn/jwt-best-practices/#10-how-to-work-with-the-signature

Within the OpenID Connect autodiscovery document there is a property called id_token_signing_alg_values_supported that contains a list of allowed signature algorithms for the tokens: https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata

Currently this library does not check this value when validating the signature in \Jumbojett\OpenIDConnectClient::verifyJWTsignature(). Is that deliberate? If not; are you open to introduce support for this? I'm willing to try and create a pull request, but not if you don't plan to support this anyway :)

[timoschinkel]

Looking at the other issues, this might be a duplicate of #341