add IBCClientHeader func for endpoint and update tests

[expertdicer]

Description

closes: #4003

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[DimitrisJim]

heyo @expertdicer, sorry for delay, we discussed this last week and Colin left a comment on the issue w.r.t how we want to add this, should be minor tweaks!

Let us know if you want us to push these final changes, thanks again!

[colin-axner]

I can push the changes next week if you don't get a chance before then (converting to draft for now)

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (1c0c756) 81.59% compared to head (ace9423) 81.54%.
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5493      +/-   ##
==========================================
- Coverage   81.59%   81.54%   -0.05%     
==========================================
  Files         199      199              
  Lines       15167    12079    -3088     
==========================================
- Hits        12375     9850    -2525     
+ Misses       2326     1764     -562     
+ Partials      466      465       -1     

Files	Coverage Δ
...interchain-accounts/controller/keeper/handshake.go	84.15% <100.00%> (+0.45%)	⬆️
...ules/apps/27-interchain-accounts/types/metadata.go	81.01% <100.00%> (+1.21%)	⬆️
modules/apps/transfer/types/codec.go	100.00% <ø> (+13.79%)	⬆️
modules/core/03-connection/keeper/verify.go	83.63% <100.00%> (+3.27%)	⬆️
modules/core/03-connection/types/connection.go	100.00% <ø> (+10.16%)	⬆️
modules/core/04-channel/keeper/handshake.go	90.22% <100.00%> (+0.01%)	⬆️
modules/core/04-channel/keeper/timeout.go	95.15% <100.00%> (-0.25%)	⬇️
modules/core/04-channel/keeper/upgrade.go	92.06% <100.00%> (-0.41%)	⬇️
modules/apps/transfer/keeper/relay.go	90.32% <60.00%> (-0.62%)	⬇️
modules/apps/transfer/types/packet.go	88.88% <50.00%> (-3.80%)	⬇️
... and 1 more

... and 160 files with indirect coverage changes

[colin-axner]

given that we don't currently store historical headers (we only keep the latest committed header), we could drop the header arg and assume that. But fine as is, might be more flexible for future testing scenarios

[colin-axner]

Nice!

[colin-axner]

Thanks @crodriguezvega for applying the changes!