Multi-tenancy support

[mancze]

Feature description

We have an multi-tenant application. Every tenant of our application authenticates using oauth2 against a different OIDC RP. It would be nice if Micronaut security natively supported this case for oauth2 authentication. Every tenant should be able to resolve their own OpenIdClient/OauthClient, OpenIdProviderMetadata and all related tenat-specific configuration/beans.

Our custom implementation got a bit awkward after micronaut-security:4.8.0 introduced ReactiveJsonWebTokenSignatureValidator<SignedJWT> which is supposed to validate JWT token. The problem is that corresponding openid metadata (with matching JWKS uri to be used for a given tenant) is not being passed down to the validateSignature method. They have to be inferred from the context instead.