Add #[kani::unstable] attribute

[celinval]

Description of changes:

Add the #[unstable] attribute that will allow us to mark unstable APIs. This PR only adds the attributes and we still need to add the option to enable them and to error out if the user is trying to use an unstable item without explicitly opting-in.

Resolved issues:

Related to #2279

Related RFC:

#2281

Call-outs:

This PR is not yet enforcing the correct format which will follow the one described in the RFC:

#[kani::unstable(feature="<IDENTIFIER>", issue="<TRACKING_ISSUE_NUMBER>", reason="<OPTIONAL_DESCRIPTION>")]
pub fn unstable_api() {}

Testing:

• How is this change tested? Manually tested

• Is this a refactor change? No

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.

[jaisnan]

Could you add an example of what the usage of this attribute might look like in the description?

[celinval]

@jaisnan right now this PR is not enforcing anything, including format. I'll add that to my follow up PR. I did ask the format in the description as requested.