Add support for creating dynamic EnumTypes.
Change-Id: Ic9f5b73630734848b29cc436f6c179549a8ea74a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/237219
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Joe Tsai <joetsai@google.com>