feat: Implement NoteMetadata protobuf message and NoteType enum

[igamigo]

Closes #315
Closes #314

[hackaugusto]

LGTM

[bobbinth]

Thank you! Looks good! I left a few comments inline. Most are minor - but three is one in which I wonder if using enum for note type was actually a good idea.

[bobbinth]

Should these values be aligned with values here. If so, I wonder if there is a good way to keep them consistent (maybe using enum here was not such a good idea?).

[igamigo]

Yeah, not sure about this. There cannot be protobuf enums that don't start with 0 so it's either creating a phantom variant, creating a custom mapping function (from protobuf's note type to base note type), or just removing it altogether and using a fixed32. For making sure they stay synced I can add a test that asserts this.

[igamigo]

On second thought it's not worth the added complexity if we can just convert the note type number. Removed in favor of a fixed32.

[bobbinth]

Agreed! I came to the same conclusion. The only question - should we use int32 instead? (this way, encoding would take 1 byte instead of 4, but otherwise should be the same).

[igamigo]

Changed to uint32. Seemed similar in terms of encoding low values but marginally more correct for the use case.

[bobbinth]

Looks good! Thank you! In addition to the small comment I left inline, I think we should change the type of note_type in protobuf message from fixed32 to int32.