chore: De-duplicate code to create BlockHeader for genesis and view

[tayfunelmas]

We address a TODO item to use BlockHeader::new when creating a BlockHeader in two other places (today these places use similar logic to choose the right BlockHeader version and contruct the inner structs based on the protocol version).

1. When generating BlockHeader from BlockHeaderView.
2. When generating the genesis BlockHeader for the genesis block.

For this, we add BlockHeader::new_impl to contain the common code and have two other wrapper functions to call new_impl for the use cases above: BlockHeader::from_view and BlockHeader::genesis.

[codecov]

Codecov Report

Attention: Patch coverage is 99.19028% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.74%. Comparing base (99ecfa4) to head (f789307).

Files with missing lines	Patch %	Lines
core/primitives/src/block_header.rs	99.53%	0 Missing and 1 partial ⚠️
core/primitives/src/views.rs	96.87%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12109      +/-   ##
==========================================
+ Coverage   71.69%   71.74%   +0.04%     
==========================================
  Files         825      825              
  Lines      165834   165697     -137     
  Branches   165834   165697     -137     
==========================================
- Hits       118902   118884      -18     
+ Misses      41751    41635     -116     
+ Partials     5181     5178       -3     

Flag	Coverage Δ
backward-compatibility	0.17% <0.00%> (+<0.01%)	⬆️
db-migration	0.17% <0.00%> (+<0.01%)	⬆️
genesis-check	1.26% <0.00%> (+<0.01%)	⬆️
integration-tests	38.91% <99.19%> (+0.06%)	⬆️
linux	71.43% <99.19%> (+0.05%)	⬆️
linux-nightly	71.34% <99.19%> (+0.07%)	⬆️
macos	53.73% <95.54%> (-0.52%)	⬇️
pytests	1.57% <0.00%> (+<0.01%)	⬆️
sanity-checks	1.38% <0.00%> (+<0.01%)	⬆️
unittests	65.53% <95.54%> (+0.15%)	⬆️
upgradability	0.21% <0.00%> (+<0.01%)	⬆️

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[Longarithm]

This is great, thank you!

[Longarithm]

We could probably unite arguments in BlockHeaderInnerLite, BlockHeaderInnerRestV1 (?) and the rest earlier to reduce number of arguments.

[tayfunelmas]

It would be great to reduce the number of arguments (it is way too many now), but not sure how to do that at this point, let me submit this and I will reconsider that in a new PR.