Add block_by_hash RPC endpoint and tests

[hansl]

Links #832.

Implements the block_by_hash RPC which returns an optional block.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[codecov-commenter]

Codecov Report

Merging #1089 (1ff611c) into master (904862c) will decrease coverage by 0.0%.
The diff coverage is 33.3%.

@@           Coverage Diff            @@
##           master   #1089     +/-   ##
========================================
- Coverage    64.6%   64.6%   -0.1%     
========================================
  Files         239     240      +1     
  Lines       20695   20727     +32     
========================================
+ Hits        13387   13405     +18     
- Misses       7308    7322     +14     

Impacted Files	Coverage Δ
rpc/src/client.rs	4.9% <0.0%> (-0.2%)	⬇️
rpc/src/client/bin/main.rs	0.3% <0.0%> (-0.1%)	⬇️
tools/rpc-probe/src/common.rs	0.0% <0.0%> (ø)
rpc/src/endpoint/block_by_hash.rs	10.0% <10.0%> (ø)
rpc/src/method.rs	24.7% <50.0%> (+0.9%)	⬆️
rpc/tests/kvstore_fixtures.rs	98.1% <100.0%> (+<0.1%)	⬆️
tendermint/src/block/commit_sig.rs	78.4% <0.0%> (-2.0%)	⬇️
light-client-verifier/src/types.rs	38.2% <0.0%> (-0.3%)	⬇️
tendermint/src/block/header.rs	99.2% <0.0%> (-0.1%)	⬇️
proto/src/lib.rs	100.0% <0.0%> (ø)
... and 6 more

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[thanethomson]

This looks great, thanks @hansl! 🎉 Just one minor nit regarding the changelog entry, and I think you need to do a cargo fmt, then we should be good to merge this.

[hansl]

Merged your suggestion, and ran cargo fmt. Thanks!