Relocate `benchttp/cli` to `benchttp/engine`

[moreirathomas]

Description

For the foreseeable future, Benchttp will be meant to be used through the command line only.

We expect to improve the DX b having the cli code inside the main repository .

Tasks

• relocate benchttp/cli code inside this repository
• try to keep the best git history as possible

[moreirathomas]

Pros/cons analysis:

Having all code under the same repo.

Pros

• Unified CI/CD
• Automatic integration of the core's new features and fixes inside the cli
• Clearer PR and issue tracking
• May help refactor (planned and emergent) by virtue of having all the code in one place
• Matches more closely our new use case : Benchttp is a command line tool, there's not supported front end other than the cli. The CI job already depends on the cli.

Cons

• Not as straight forward boundaries, but honestly independent packages should solve this issue

Keeping the cli and core code separated

Pros

• Clear consumer/provider split, enforces I/O uncoupling
• May evolve at a difference pace than the core
• Would be good if we continue to support another front end than the cli (redoing the split is easy however)

Cons

• No automatic features and fixes update
• Must juggle two repositories
• Artificial cut given the cli ought to be our only way of interacting with the core