feat: initialize cctx gateway interface

[skosito]

Description

Adds new CCTXGateway interface and first part of implementation for observers and zevm. Check issue and docs for more details.

Closes: #2276

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Include instructions and any relevant details so others can reproduce.

• Tested CCTX in localnet
• Tested in development environment
• Go unit tests
• Go integration tests
• Tested via GitHub Actions

Checklist:

• I have added unit tests that prove my fix feature works

[codecov]

Codecov Report

Attention: Patch coverage is 93.68421% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 69.27%. Comparing base (8be6ce1) to head (1f0638a).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2291      +/-   ##
===========================================
+ Coverage    69.26%   69.27%   +0.01%     
===========================================
  Files          274      276       +2     
  Lines        16537    16563      +26     
===========================================
+ Hits         11454    11474      +20     
- Misses        4599     4604       +5     
- Partials       484      485       +1     

Files	Coverage Δ
x/crosschain/keeper/cctx_gateway_observers.go	100.00% <100.00%> (ø)
x/crosschain/keeper/cctx_gateway_zevm.go	100.00% <100.00%> (ø)
x/crosschain/keeper/initiate_outbound.go	100.00% <100.00%> (ø)
x/crosschain/keeper/msg_server_vote_inbound_tx.go	92.00% <33.33%> (-3.84%)	⬇️
app/app.go	2.16% <0.00%> (-0.02%)	⬇️

[skosito]

@lumtis just a draft PR for now to see if it's on right track, what do you think? most of the logic is copied and will be refined further with adding ValidateOutbound and ValidateInbound.
or scope of this issue is bigger than this?

[lumtis]

I think the direction is good

[lumtis]

Just changing review, I inadvertently approved on the last one

[lumtis]

[lumtis]

Need to run make generate

[kingpinXD]

lgtm , minor comments have a look before merging