API: Explicitly set the x-algorand-format for BlockHeaderResponse to 'Bloc…

[gmalouf]

Summary

Explicitly set the x-algorand-format for BlockHeaderResponse to 'BlockHeaderResponse'. This allows for downstream code-generators to differentiate between the slightly different msgpack responses on /v2/blocks/{round} and /v2/blocks/{round}/header

Test Plan

Existing checks should all pass.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.75%. Comparing base (e60d3dd) to head (c1930ee).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6247      +/-   ##
==========================================
+ Coverage   49.25%   51.75%   +2.50%     
==========================================
  Files         644      644              
  Lines       86521    86521              
==========================================
+ Hits        42617    44781    +2164     
+ Misses      41040    38872    -2168     
- Partials     2864     2868       +4     

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[algorandskiy]

Corresponding generator PR: algorand/generator#85