MCP: Add Resource and Prompt Abstract Classes

[n7studios]

Summary

Adds resource and prompt abstract classes, for later PRs to register resources and prompts with MCP that AI clients can utilise for managing the Kit WordPress Plugin.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)