[pigeon] Use a const for custom type ids for gobject generated files (#156100)

[JesseRiemens]

Adds a custom type identifier to generated gobject headers for the user. Calling fl_value_new_custom_object is now possible with that constant.

This fixes flutter/flutter#156100

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[JesseRiemens]

The FLValue headers in some places use int, and in some places use uint8_t:

// fl_value.h:303
FlValue* fl_value_new_custom(int type, // uses int
                             gconstpointer value,
                             GDestroyNotify destroy_notify);

// fl_standard_message_codec.h:148
gboolean fl_standard_message_codec_write_value(FlStandardMessageCodec* codec,
                                               GByteArray* buffer,  // uses GByteArray*, which is used (subclassed) in generated files:
                                               FlValue* value,
                                               GError** error);
// core_tests.gen.cc:2422
static gboolean
core_tests_pigeon_test_message_codec_write_core_tests_pigeon_test_an_enum(
    FlStandardMessageCodec* codec, GByteArray* buffer, FlValue* value,
    GError** error) {
  uint8_t type = core_tests_pigeon_test_an_enum_type_id; // appends uint8_t to buffer
  g_byte_array_append(buffer, &type, sizeof(uint8_t));
  return fl_standard_message_codec_write_value(codec, buffer, value, error);
}

I think this might be related to #152916. Maybe it is best to leave it as is, and for this PR use int for the types.

[stuartmorgan-g]

The FLValue headers in some places use int, and in some places use uint8_t:

Where is the header using uint8_t for the type? I'm seeing in your snippet:

• A header declaration that uses int for the type.
• A header declaration that doesn't have the type at all.
• A file that's not an engine header.

[JesseRiemens]

My mistake, it's not the FlValue header that uses uint8_t, but only the generated code using the engine header.
I did not fully understand how it worked, but now I see that the generated dart code at the other side also uses uint8:

class DartGenerator extends StructuredGenerator<InternalDartOptions> {
  (...)
  void writeGeneralCodec() {
    (...)
          indent.writeln('buffer.putUint8(4);');

dart_generator.dart:432

That makes more sense to me now. I guess that means we can keep it like it is.