rpc: support EXISTS operator in /tx_search query

[melekes]

Closes #4763

[auto-comment]

👋 Thanks for creating a PR!

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Wrote tests
• Updated CHANGELOG_PENDING.md
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments
• Re-reviewed Files changed in the Github PR explorer
• Applied Appropriate Labels

Thank you for your contribution to Tendermint! 🚀

[codecov]

Codecov Report

Merging #4979 into master will decrease coverage by 0.06%.
The diff coverage is 36.84%.

@@            Coverage Diff             @@
##           master    #4979      +/-   ##
==========================================
- Coverage   62.77%   62.70%   -0.07%     
==========================================
  Files         175      175              
  Lines       15926    15938      +12     
==========================================
- Hits         9997     9994       -3     
- Misses       5080     5091      +11     
- Partials      849      853       +4     

Impacted Files	Coverage Δ
state/txindex/kv/kv.go	66.02% <36.84%> (-1.90%)	⬇️
blockchain/v2/reactor.go	35.92% <0.00%> (-2.97%)	⬇️
blockchain/v0/reactor.go	62.21% <0.00%> (-0.93%)	⬇️
blockchain/v0/pool.go	78.66% <0.00%> (-0.32%)	⬇️
p2p/pex/pex_reactor.go	80.95% <0.00%> (+0.52%)	⬆️
behaviour/reporter.go	57.69% <0.00%> (+9.20%)	⬆️

[tac0turtle]

do we have somewhere in the docs explaining the indexer? if so we should add this there