Add staked balances query for easier frontend integration

[JakeHartnell]

Closes #182

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (25774f0) 86.26% compared to head (546403a) 86.27%.

Files	Patch %	Lines
...acts/provider/native-staking-proxy/src/contract.rs	85.71%	0 Missing and 1 partial :warning:

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #183   +/-   ##
=======================================
  Coverage   86.26%   86.27%           
=======================================
  Files          81       81           
  Lines       14253    14269   +16     
  Branches    14253    14269   +16     
=======================================
+ Hits        12295    12310   +15     
  Misses       1389     1389           
- Partials      569      570    +1     

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[ethanfrey]

What's missing here? I guess a test with multiple delegations?

If you rebase it and take it out of draft, I'm happy to review and merge