Extend DA layer client interface

[tzdybal]

• Add CheckBlockAvailability method (resolvesDefine Universal Data Availability Check API #61)
• Add BlockRetriever interface (resolvesDefine universal Data Availability Retrieval API #62)
• Extend mock implementation (resolvesImplement Mock Data Availability client #63)

[codecov-commenter]

Codecov Report

Merging #83 (e37f945) into main (2e8849e) will increase coverage by 0.67%.
The diff coverage is 64.28%.

@@ Coverage Diff @@## main #83 +/- ##
==========================================
+ Coverage 57.15% 57.82% +0.67% 
==========================================
Files 19 21 +2 Lines 1230 1252 +22 ==========================================
+ Hits 703 724 +21 - Misses 428 432 +4 + Partials 99 96 -3 

Impacted Files	Coverage Δ
types/hashing.go	0.00% <0.00%> (ø)
da/lazyledger/lazyledger.go	10.00% <20.00%> (+1.37%)	⬆️
da/mock/mock.go	100.00% <100.00%> (ø)
state/executor.go	72.59% <100.00%> (+2.08%)	⬆️
store/store.go	66.66% <100.00%> (+2.38%)	⬆️

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[Wondertan]

Question: What's the rationale to use an array and not slice?

[tzdybal]

• crypto/sha3 returns array
• we used arrays in structs, because hash is fixed size (logically)

[Wondertan]

we used arrays in structs, because hash is fixed size (logically)
Totally, but in practice, we don't use fixed-size arrays and kinda forcing users to always slice it themselves, but that is ok as long you ok with that.