Subtype Id registration moved to its own package (#1416)

* Allow repo packages to register subtypes and id prefixes. 
* Refactor out request authentication logic to a new package as a sibling to handler.

internal/errors/code_test.go

@@ -112,6 +112,11 @@ func TestCode_Both_String_Info(t *testing.T) {
 			c:    JobAlreadyRunning,
 			want: JobAlreadyRunning,
 		},
+		{
+			name: "SubtypeAlreadyRegistered",
+			c:    SubtypeAlreadyRegistered,
+			want: SubtypeAlreadyRegistered,
+		},
 		{
 			name: "InvalidDynamicCredential",
 			c:    InvalidDynamicCredential,