Give callbacks its own go.mod (backport #4341)

[mergify]

This is an automatic backport of pull request #4341 done by Mergify.
Cherry-pick of a18159f has failed:

On branch mergify/bp/release/v7.3.x/pr-4341
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  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	new file:   .github/workflows/callbacks.yml
	modified:   modules/apps/callbacks/callbacks_test.go
	modified:   modules/apps/callbacks/fee_transfer_test.go
	new file:   modules/apps/callbacks/go.mod
	new file:   modules/apps/callbacks/go.sum
	modified:   modules/apps/callbacks/ibc_middleware_test.go
	modified:   modules/apps/callbacks/ica_test.go
	modified:   modules/apps/callbacks/testing/simapp/app.go
	modified:   modules/apps/callbacks/testing/simapp/contract_keeper.go
	modified:   modules/apps/callbacks/testing/simapp/encoding.go
	modified:   modules/apps/callbacks/transfer_test.go
	modified:   modules/apps/callbacks/types/callbacks_test.go
	modified:   modules/apps/callbacks/types/events_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   modules/apps/callbacks/export_test.go
	both modified:   modules/apps/callbacks/ibc_middleware.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[srdtrk]

Important note: We still use replace github.com/cosmos/ibc-go/v7 => ../../../ in callbacks mw's go.mod because there are some changes we've added to release/v7.3.x since v7.3.0-rc0 that are required for this integration, namely:

• testing: add function field to override SendMsgs (backport #3767) #4449
• Addition of this field to ibcmock in feat(callbacks): adr8 implementation (backport #3939) #4432. Maybe I should open a PR to move these to contract_keeper.go in both backport and main?

Regardless, a second release candidate is required for the replace statement to be removed. The second release candidate must be tagged after this PR is merged.

[crodriguezvega]

I added this from #4355.

[crodriguezvega]

The linter is causing more trouble than anything else, I will take it out.