Add validator_delegations query

[freak12techno]

No description provided.

[keppel]

Looking good, but since this query can be pretty expensive for a validator with a lot of delegators, in the future we'll want to support pagination here.

Options include:

• Pagination for just this query (would need to provide a limit and a start_key since an integer offset wouldn't make sense in the underlying map).
• More general support for query pagination (eg. a store error after some number of reads on the resolving node triggering the partial response)
• A feature flag for enabling expensive queries for use cases where you're only querying a local node.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (11701c3) 58.89% compared to head (f0bcb96) 58.85%.
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #249      +/-   ##
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- Coverage    58.89%   58.85%   -0.04%     
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  Files           84       84              
  Lines        15744    15748       +4     
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- Hits          9272     9269       -3     
- Misses        6472     6479       +7     

Files	Coverage Δ
src/coins/staking/mod.rs	61.94% <0.00%> (-0.38%)	⬇️

... and 1 file with indirect coverage changes