Feat(plugins): Add "unique_keys" in avdschema.

[ClausHolbechArista]

Change Summary

Add "unique_keys" in avdschema.

Component(s) name

AVD Schema validation

Proposed changes

Support unique_keys in the AVD Meta schema so we can validate uniqueness of one or more keys which is not the primary_key.
Also supports checking uniqueness across nested data models by using dot-notated variable path.

Limitation: If the test for nested keys overlap with the primary_key of an underlying dict, it will emit errors from both checks. This should be solved by removing primary_key on the nested dict, and just set required: true on the key instead.

How to test

Added unit tests for both positive and negative cases.
Also tested manually with cv_pathfinder_regions where we want unique_keys: [name, sites.name" to ensure unique site names across multiple regions.

Checklist

User Checklist

• N/A

Repository Checklist

• My code has been rebased from devel before I start
• I have read the CONTRIBUTING document.
• My change requires a change to the documentation and documentation have been updated accordingly.
• I have updated molecule CI testing accordingly. (check the box if not applicable)

[gmuloc]

LGTM

[carlbuchmann]

LGTM