Add Virtual MCP Server proposal

This proposal introduces a Virtual MCP Server that aggregates multiple
MCP servers from a ToolHive group into a single unified interface.

Key features:
- Leverages existing ToolHive groups for backend management
- Uses existing ToolsFilter and ToolOverride constructs
- Supports per-backend authentication strategies
- Enables composite tools for cross-service workflows
- Maintains full MCP protocol compatibility

The Virtual MCP Server will simplify client integration by providing a
single connection point while handling the complexity of multiple backend
authentication requirements and tool namespace management.

Signed-off-by: Juan Antonio Osorio <ozz@stacklok.com>

Author: JAORMX