Deprecate standalone kani, add support for cargo-kani for every call

[jaisnan]

Description of changes:

Switch to cargo-kani for all commands, and deprecate usage of standalone kani.

Call-outs:

Need to raise an issue or call out in documentation that we don't support standalone kani as a proof runner i.e we require that the extension be run on a crate with a declared cargo.toml. This is similar to how rust-analyzer handles it too, rust-lang/rust-analyzer#8818.

Testing:

• How is this change tested? Manual testing

• Is this a refactor change? Yes

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[adpaco-aws]

It makes sense for the extension to only use cargo kani commands, but I have a few questions.

[adpaco-aws]

Thanks, @jaisnan !