align block header with protobuf schema

[illia-malachyn]

Closes: #765

[illia-malachyn]

Should I come up with some generator for places where I can use "dummy"?

[peterargue]

yea, you can have it generate random data of some length. not sure what's available in this repo, but here's the implementations in flow-go
https://github.com/onflow/flow-go/blob/0dde73002b873c30a94e995f80b97ad2cc42355e/utils/unittest/fixtures.go#L1068

[illia-malachyn]

I added BytesGenerator to this PR

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.82677% with 18 lines in your changes missing coverage. Please review.

Project coverage is 56.78%. Comparing base (7b58582) to head (d070f5e).
Report is 67 commits behind head on master.

Files with missing lines	Patch %	Lines
access/grpc/convert/convert.go	82.60%	8 Missing and 8 partials ⚠️
test/entities.go	94.28%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
+ Coverage   52.92%   56.78%   +3.86%     
==========================================
  Files          35       35              
  Lines        5580     6098     +518     
==========================================
+ Hits         2953     3463     +510     
+ Misses       2231     2136      -95     
- Partials      396      499     +103     

Flag	Coverage Δ
unittests	56.78% <85.82%> (+3.86%)	⬆️

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