CheckToken checks the 'authentication' header of incoming gRPC metadata in ctx.

ExtractAuthInfo returns an `AuthInfo` with the data extracted from `ctx`.

RPCCredentials can be used with grpc.WithPerRPCCredentials to create a grpc.DialOption that inserts the supplied token for authentication with a Gitaly server.

RPCCredentialsV2 can be used with grpc.WithPerRPCCredentials to create a grpc.DialOption that inserts an HMAC token with the current timestamp for authentication with a Gitaly server.

AuthInfo contains the authentication information coming from a request.