Add proposal for operator registry integration via Gateway API

This proposal enables the ToolHive Kubernetes operator to automatically
populate the MCP registry with externally accessible endpoints for
MCPServer, MCPRemoteProxy, and VirtualMCPServer resources.

Key features:
- Gateway API HTTPRoute annotation-based discovery (primary approach)
- Direct URL annotation fallback (for non-Gateway API users)
- Explicit opt-in model for administrator control
- Upstream MCP registry format with ToolHive publisher extensions
- Kubernetes Events for discovery tracking

The approach maintains clear separation between ingress configuration
and MCP resource definitions, following ToolHive's design principles.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: JAORMX