Add IssueBundle::note_commitments to expose extracted note commitments

[dmidem]

This adds IssueBundle::note_commitments() to expose extracted note commitments directly as pallas::Base values.

This is useful in Zebra to recompute issuance-related commitments without the current to_bytes/from_bytes workaround:

https://github.com/QED-it/zebra/blob/2634fd662a4eaca6df901c82b1bc05d2982636d7/zebra-chain/src/orchard_zsa/issuance.rs#L43

See also the related Zebra PR review comment:

QED-it/zebra#111 (comment)

[PaulLaux]

Looks good, but a possitive+negative(empty bundle) test is required to finilize. For possitive test, you can add it to the big "happy flow" test if possible.

[dmidem]

Looks good, but a possitive+negative(empty bundle) test is required to finilize. For possitive test, you can add it to the big "happy flow" test if possible.

Done. I added a positive test for note_commitments() to the existing issue_bundle_verify_with_issued_assets test - it ensures note_commitments returns a correct number of non-zero, unique pallas::Base values. And I also added a negative test issue_bundle_note_commitments_empty_bundle that ensures note_commitments() returns no items for an empty bundle.