Make GRPCApplicationProtocolIdentifier internal (#665)

Sources/GRPC/TLSVerificationHandler.swift

@@ -20,7 +20,7 @@ import NIOTLS
 import Logging
 
 /// Application protocol identifiers for ALPN.
-public enum GRPCApplicationProtocolIdentifier: String, CaseIterable {
+internal enum GRPCApplicationProtocolIdentifier: String, CaseIterable {
   // This is not in the IANA ALPN protocol ID registry, but may be used by servers to indicate that
   // they serve only gRPC traffic. It is part of the gRPC core implementation.
   case gRPC = "grpc-ext"