fix(header)!: use tmjson encoding for Commit field as well

[renaynay]

Fixes #3918

Please note: this PR is BREAKING as header encoding has changed.

I don't see a reason we weren't doing this to begin with as we tmjson encode the other "tendermint" fields.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 45.10%. Comparing base (2469e7a) to head (5ebb191).
Report is 378 commits behind head on main.

Files with missing lines	Patch %	Lines
header/header.go	50.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3925      +/-   ##
==========================================
+ Coverage   44.83%   45.10%   +0.27%     
==========================================
  Files         265      307      +42     
  Lines       14620    21846    +7226     
==========================================
+ Hits         6555     9854    +3299     
- Misses       7313    10912    +3599     
- Partials      752     1080     +328     

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[Wondertan]

We should tmjson the whole ExtendedHeader altogether, without aliasing and individual calls for each field