tendermint-testgen: Add app_hash field to Header

[Farhad-Shabani]

Closes: #1343

Use case: informalsystems/hermes#3566

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[codecov-commenter]

Codecov Report

Merging #1344 (b79e0d3) into main (c2b5c9e) will increase coverage by 1.0%.
Report is 2 commits behind head on main.
The diff coverage is 48.1%.

❗ Current head b79e0d3 differs from pull request most recent head c621426. Consider uploading reports for the commit c621426 to get more accurate results

@@           Coverage Diff           @@
##            main   #1344     +/-   ##
=======================================
+ Coverage   59.3%   60.3%   +1.0%     
=======================================
  Files        273     272      -1     
  Lines      27095   26528    -567     
=======================================
- Hits       16068   16010     -58     
+ Misses     11027   10518    -509     

Files Changed	Coverage Δ
proto/src/lib.rs	100.0% <ø> (ø)
testgen/src/header.rs	76.8% <23.5%> (-6.3%)	⬇️
p2p/src/secret_connection.rs	85.5% <50.0%> (-0.3%)	⬇️
testgen/src/light_block.rs	86.5% <100.0%> (-0.5%)	⬇️

... and 16 files with indirect coverage changes

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