APIs for mobile SDKs

[str4d]

This PR only adds one of the needed APIs for the mobile SDKs; due to also refactoring the traits, I decided to split the other changes (which are themselves non-trivial) to a follow-on PR.

[nuttycom]

utACK, essentially move-only.

[codecov]

Codecov Report

Attention: Patch coverage is 31.14754% with 42 lines in your changes are missing coverage. Please review.

Project coverage is 63.55%. Comparing base (b3d06ba) to head (634ebf5).

Files	Patch %	Lines
zcash_client_backend/src/data_api.rs	0.00%	24 Missing ⚠️
zcash_client_sqlite/src/lib.rs	61.29%	12 Missing ⚠️
zcash_client_backend/src/proto.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1266      +/-   ##
==========================================
- Coverage   63.57%   63.55%   -0.03%     
==========================================
  Files         121      121              
  Lines       13646    13652       +6     
==========================================
  Hits         8676     8676              
- Misses       4970     4976       +6     

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