Refactor policy validators to inherit an interface for future policies

[Will-Lo]

Summary

Issue] Briefly discuss the summary of the changes made in this
pull request in 2-3 lines.

As Openhouse begins to support more policies, it becomes beneficial to have all the validators follow a common interface, defined by PolicySpecValidator. It takes in the entire request sent for the Create/Update table request since some policies may be dependent on other properties of the table.

This PR performs a small refactor to have these classes share the same base class, and future policy definitions to follow this pattern.

Changes

• Client-facing API Changes
• Internal API Changes
• Bug Fixes
• New Features
• Performance Improvements
• Code Style
• Refactoring
• Documentation
• Tests

For all the boxes checked, please include additional details of the changes made in this pull request.

Testing Done

• Manually Tested on local docker setup. Please include commands ran, and their output.
• Added new tests for the changes made.
• Updated existing tests to reflect the changes made.
• No tests added or updated. Please explain why. If unsure, please feel free to ask for help.
• Some other form of testing like staging or soak time in production. Please explain.

For all the boxes checked, include a detailed description of the testing done for the changes made in this pull request.

Additional Information

• Breaking Changes
• Deprecations
• Large PR broken into smaller PRs, and PR plan linked in the description.

For all the boxes checked, include additional details of the changes made in this pull request.

[chenselena]

mostly looks good, just 2 small suggestions. thanks for the refactor William!

[chenselena]

lgtm!