feat: 📝 implement cometstub crate (work-in-progress)

[cratelyn]

see:

• Tracking issue: 🤝 mock consensus engine for App tests #3588
• feat: 📝 implement cometstub crate (work-in-progress) #3597

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/!\ NOTE: this is a rolling work-in-progress.               /!\
/!\ this branch will be force-pushed frequently until it is /!\
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[hdevalence]

Two high-level comments:

1. we should avoid using the word "mock" to prevent confusion with cometmock, which is a different piece of software
2. we should have a separate crate, maybe "cometstub"/"tenderstub", that just has the functionality of producing headers (and possibly abci functionality, but no penumbra-specific code) so that this component can be more easily reused in other parts of the ecosystem (eg https://x.com/hdevalence/status/1736860951000031672 )

[cratelyn]

Two high-level comments:

1. we should avoid using the word "mock" to prevent confusion with cometmock, which is a different piece of software

@hdevalence that sounds good to me. before we accrue further review context here, and because github doesn't provide a way to switch the branch associated with a pull request, i am going to close this in favor of #3597 (katie/cometstub).

2. we should have a separate crate, maybe "cometstub"/"tenderstub", that _just_ has the functionality of producing headers (and possibly abci functionality, but no penumbra-specific code) so that this component can be more easily reused in other parts of the ecosystem (eg https://x.com/hdevalence/status/1736860951000031672 )

this also sounds like a great idea to me. if it is alright with you, i will wait until i am a little further along to split things into two crates, hopefully at which point i will have a clearer picture of how these pieces fit together.

that said, a separate library for generating headers sounds like a wise division of responsibilities.

i am happy to continue this conversation forward over in #3597!