rpc: add support for /tx endpoint

[tony-iqlusion]

Support for finding a transaction directly by its hash.

In theory this endpoint should have read-your-writes (RYW) consistency whereas /tx_search does not.

See: tendermint/tendermint#6359

[codecov-commenter]

Codecov Report

Merging #863 (2ae6986) into master (4fa065b) will increase coverage by 0.1%.
The diff coverage is 96.3%.

❗ Current head 2ae6986 differs from pull request most recent head 03860fb. Consider uploading reports for the commit 03860fb to get more accurate results

@@           Coverage Diff            @@
##           master    #863     +/-   ##
========================================
+ Coverage    67.0%   67.1%   +0.1%     
========================================
  Files         212     213      +1     
  Lines       16771   16823     +52     
========================================
+ Hits        11244   11299     +55     
+ Misses       5527    5524      -3     

Impacted Files	Coverage Δ
rpc/src/client.rs	16.9% <ø> (ø)
rpc/src/endpoint/tx_search.rs	10.0% <ø> (-6.7%)	⬇️
rpc/src/endpoint/tx.rs	50.0% <50.0%> (ø)
rpc/src/method.rs	29.2% <50.0%> (+0.9%)	⬆️
rpc/tests/parse_response.rs	98.0% <100.0%> (+0.3%)	⬆️
rpc/src/error.rs	50.4% <0.0%> (-1.5%)	⬇️
testgen/src/header.rs	78.1% <0.0%> (-0.7%)	⬇️
abci/src/server.rs	7.6% <0.0%> (+<0.1%)	⬆️
... and 4 more

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[thanethomson]

LGTM! 👍