break(terraform): return UNKNOWN for unrendered values in graph checks

[YaaraVerner]

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

Description

Added UNKNOWN graph check result for unrendered attributes.

Added a new list of unknown results to the return value of the solvers' run function.
Changed the return value of get_operation function in attribute and complex solvers to Optional[bool], None value will indicate UNKNOWN check result.
Added a new list of unknown results to the return value of the connections solvers' get_operation function.

Removed the handling of this use case in specific solvers.
Adjusted the solvers' tests to expect UNKNOWN results for those use cases.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my feature, policy, or fix is effective and works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[nimrodkor]

Very nice @YaaraVerner !
Small comments

[gruebel]

nice work, a lot to dig through 😄 added a few comments

[gruebel]

nice, great work 🥇