feat: add storage interface and support for file or db based templates

[ericsizemore]

Pull Request

Proposed Changes

Adds a new Storage interface and introduces database template support in addition to filesystem based templates. Also replaces native exceptions with library-specific exceptions.

Readiness Checklist

Author/Contributor

• You have read CONTRIBUTING
• If documentation is needed for this change, has that been included in this pull request
• run composer run-script phpunit and ensure you have test coverage for the lines you are introducing
• run composer run-script phpstan and fix any issues that you have introduced
• run composer run-script psalm and fix any issues that you have introduced
• run composer run-script phpcs:check and fix any issues that you have introduced

Reviewer

• Label as either fix, documentation, or enhancement
• Additionally label as verified or unverified