support multiple jwt issuers when validating

[StephenWithPH]

The underlying JWT library supports multiple issuers. See auth0/java-jwt#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:

ktor/ktor-server/ktor-server-plugins/ktor-server-auth-jwt/jvm/src/io/ktor/server/auth/jwt/JWTUtils.kt

Line 74 in 221f467

else -> JWT.require(algorithm).withIssuer(issuer)