add HeaderBlock

[flyinprogrammer]

No description provided.

[upgreydd]

@flyinprogrammer works, thanks. @KevinThePepper this should be merged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.23%. Comparing base (e01770c) to head (5e82c68).

Files with missing lines	Patch %	Lines
slack_blockkit/layout_block.py	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
- Coverage   86.30%   85.23%   -1.07%     
==========================================
  Files           8        8              
  Lines         292      298       +6     
==========================================
+ Hits          252      254       +2     
- Misses         40       44       +4     

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