support multiple jwt issuers when validating

The underlying JWT library supports multiple issuers. See https://github.com/auth0/java-jwt/issues/246 and https://github.com/auth0/java-jwt/blob/ee7332b023719a9007be0caf5ef7608840fc4946/lib/src/main/java/com/auth0/jwt/interfaces/Verification.java#L27-L34.

It would be nice if that was used here and pushed up to be exposed to ktor users:

https://github.com/ktorio/ktor/blob/221f467c1184da87ae1bd57ce194970246fdd7ed/ktor-server/ktor-server-plugins/ktor-server-auth-jwt/jvm/src/io/ktor/server/auth/jwt/JWTUtils.kt#L74

Start with JWTUtils.kt at the referenced validation code and compare it with Verification.java and auth0/java-jwt issue 246. Trace how issuer validation is configured for Ktor users; done means multiple JWT issuers can be supported through the exposed Ktor JWT authentication configuration.

Written by the indexing model from the issue text.