Support setting multiple issuers

[lcmgh]

Use case: Usage of the library in an environment with various OIDC providers

[06chaynes]

I'm not against adding support for multiple issuers, but it does increase the complexity a bit. A Config would either need to be shared between the issuers (might not be as helpful) or a Config would need to be provided per issuer (perhaps a bit tedious and long of tooth). Keys would also need to be stored per issuer. None of this is difficult to add but it does change the ergonomics of the library a bit.

I'll play around with this when I have a chance but would also welcome a PR to review if you are interested.

[lcmgh]

I thought about a HashMap workaround when using the "normalized" OIDC Provider URL as Key:

verifier: HashMap<String, Verifier>

But when trying to retrieve the verifier my code is not aware of the issuer url as the token was not yet decoded.