Make NoteCommitment in the API public

[natalieesk]

Zebra wants to deduplicate NoteCommitment types across zebra-chain and orchard by removing the ones in zebra-chain and using the ones in orchard.

To do this we need to expose methods for constructing NoteCommitments

[str4d]

I think the type you should likely be using is ExtractedNoteCommitment, which has constructors. Where are you needing to construct them?