feat: require compile-time flag to enable liquidator query

[toteki]

Description

This prevents nodes not specifically built for liquidators from running the inefficient liquidation_targets query.

closes: #827

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[RafilxTenfen]

I don't think we need to tell the value of LIQUIDATOR explicitly at make build and make install

[codecov-commenter]

Codecov Report

Merging #1395 (c13ed92) into main (844440c) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1395      +/-   ##
==========================================
+ Coverage   52.41%   52.44%   +0.02%     
==========================================
  Files          71       71              
  Lines        6799     6802       +3     
==========================================
+ Hits         3564     3567       +3     
  Misses       2966     2966              
  Partials      269      269              

Impacted Files	Coverage Δ
x/leverage/keeper/grpc_query.go	64.17% <100.00%> (+0.54%)	⬆️

[robert-zaremba]

Documentation is missing: both in the spec directory and release notes.