Add protobuf representation for transaction proposals

[nuttycom]

In order to make complete information about proposed transactions readily available to wallets, it's necessary to provide a mechanism to pass a TransactionProposal across the FFI. This PR adds a Protobuf representation for proposed transactions and implements conversions back and forth between the protobuf representation and the native type.

Closes #1093.

[codecov]

Codecov Report

Attention: 142 lines in your changes are missing coverage. Please review.

Comparison is base (2d6a02e) 70.14% compared to head (c5f48f5) 69.79%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
zcash_client_backend/src/proto.rs	65.83%	55 Missing ⚠️
zcash_client_backend/src/zip321.rs	12.82%	34 Missing ⚠️
...ent_backend/src/data_api/wallet/input_selection.rs	53.19%	22 Missing ⚠️
zcash_client_backend/src/proto/proposal.rs	0.00%	13 Missing ⚠️
zcash_client_sqlite/src/wallet.rs	64.51%	11 Missing ⚠️
zcash_client_backend/src/data_api.rs	0.00%	4 Missing ⚠️
zcash_client_sqlite/src/lib.rs	50.00%	2 Missing ⚠️
zcash_client_backend/src/data_api/error.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #891      +/-   ##
==========================================
- Coverage   70.14%   69.79%   -0.35%     
==========================================
  Files         141      142       +1     
  Lines       13674    13959     +285     
==========================================
+ Hits         9591     9743     +152     
- Misses       4083     4216     +133     

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[pacu]

Folks, do you have an expected deadline for accepting comments on this?
Just prioritizing here 😅

[nuttycom]

I would really like to get this in to an SDK release immediately following Spend-before-Sync, but the proposal to allow using ZIP-317 fees without displaying the fees to the user up front takes the pressure off a little bit. But ideally wallet devs would be able to adopt SbS and the proposal system simultaneously.

…

On Fri, Aug 18, 2023 at 9:54 AM Pacu ***@***.***> wrote: Folks, do you have an expected deadline for accepting comments on this? Just prioritizing here 😅 — Reply to this email directly, view it on GitHub <#891 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAGXR5Z5D5J5UY3GVZEHDLXV6F4HANCNFSM6AAAAAA3I32QNU> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[pacu]

Left non-blocking comments on PR.

1. possible feature request: create proposals from ZIP-321 string-serialized requests
2. Evaluate ways to make it hard (or better impossible) for Proposal's payment request property to be out of sync of a Proposal instance.
3. Consider adding "privacy policy" to the transaction proposal similar as the ones documented in z_sendmany and z_shieldcoinbase on Zcashd.

[str4d]

utACK 7aab6fd with a few nits.

[daira]

utACK with typo fix suggestions and a non-blocking comment.

[str4d]

Re-utACK c5f48f5

[daira]

Post-hoc ACK