hpb: make ExtensionNumber inside ExtensionIdentifier accessible in a constexpr context

We expunge the emission of the ExtensionIdentifier in the cc and `inline constexpr` it in the .h.

The extension number is fed into the constexpr ExtensionIdentifier ctor and now the id can be requested at compile time (e.g. constexpr auto extension_number = hpb::ExtensionNumber(your_extension_name_here)).

PiperOrigin-RevId: 740749472