Add RFC for Skills Support in ToolHive Registry Server (MVP)

This RFC proposes adding Agent Skills support to the registry server,
enabling discovery and cataloging through the existing infrastructure.

Key aspects:
- Skills integrated into subregistries alongside MCP servers
- Upstream spec-compliant extension API (/v0.1/x/dev.toolhive/skills)
- Support for both OCI and Git distribution methods
- Database schema with 4 new tables
- Basic search functionality for MVP

Author: JAORMX