Feature: Add an in-memory wallet backend framework

[nuttycom]

This sets up the general framework; most of the implementation is todo!().

Closes #1414.

[codecov]

Codecov Report

Attention: Patch coverage is 1.68067% with 117 lines in your changes are missing coverage. Please review.

Project coverage is 63.09%. Comparing base (f7c29f0) to head (fc757b1).
Report is 38 commits behind head on main.

❗ Current head fc757b1 differs from pull request most recent head 29e2043. Consider uploading reports for the commit 29e2043 to get more accurate results

Files	Patch %	Lines
zcash_client_backend/src/data_api/mem_wallet.rs	0.00%	115 Missing ⚠️
zcash_client_sqlite/src/lib.rs	50.00%	1 Missing ⚠️
zcash_proofs/src/circuit/sprout/input.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1293      +/-   ##
==========================================
- Coverage   63.23%   63.09%   -0.15%     
==========================================
  Files         124      122       -2     
  Lines       14507    14058     -449     
==========================================
- Hits         9174     8870     -304     
+ Misses       5333     5188     -145     

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[oxarbitrage]

I think it can be a good to create a list of the minimal requirements we consider needed to merge this PR while leave the rest of the functions to make individual pull requests against the main repo ?

[str4d]

Per the meeting today, this PR needs to be rebased on main and squashed down. Then once we're happy with it as a framework, we can merge it, and then implement its internals by migrating tests over (#1415) after the test framework has been migrated {#1412).