t8n: Include empty requests[Hash] in JSON output

[pdobacz]

Fixes #1062.

Two open items:
1/ the t8n.cpp code is quite rudimentary, but I don't want to overdo it, considering it's going to get an overhaul when implementing EIPs 7251/6110/7002. This is just enough to satisfy the t8n format required by EIP-7685
2/ I've spent a lot of time trying to find a CMakeLists setup that works. I have no idea if this is proper, I only superficially caught up with cmake docs. Also not sure if the proposed placement of sha256.xpp is OK.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.23%. Comparing base (dad97f6) to head (82c945b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1064      +/-   ##
==========================================
+ Coverage   94.22%   94.23%   +0.01%     
==========================================
  Files         158      158              
  Lines       17066    17081      +15     
==========================================
+ Hits        16081    16097      +16     
+ Misses        985      984       -1     

Flag	Coverage Δ
eof_execution_spec_tests	17.79% <0.00%> (-0.02%)	⬇️
ethereum_tests	26.50% <0.00%> (-0.03%)	⬇️
ethereum_tests_silkpre	19.10% <0.00%> (-0.02%)	⬇️
execution_spec_tests	20.21% <0.00%> (-0.02%)	⬇️
unittests	89.29% <100.00%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
test/t8n/t8n.cpp	87.79% <100.00%> (+1.80%)	⬆️