RFC: Add tool definitions to publisher extensions

[JAORMX]

Summary

Port proposal from toolhive PR #2733.

This RFC proposes adding comprehensive tool metadata to the existing ToolHive publisher extension (io.github.stacklok) in the upstream MCP Registry format. This extends beyond simple tool names to include full MCP tool definitions with descriptions, input/output schemas, and annotations.

What's Changing

• Adds tool_definitions field to the existing io.github.stacklok publisher extension
• Stores full MCP tool metadata: name, title, description, inputSchema, outputSchema, annotations
• Maintains backward compatibility with existing tools string array

Use Cases

1. Enhanced tooling output - thv list can show tool descriptions
2. API contract breakage detection - Compare schemas to detect breaking changes in server updates
3. Client integrations - IDEs/editors can display tool signatures and validate inputs

Technical Details

• Reuses existing extension namespace (no new namespaces)
• Aligns with MCP specification tool schema
• Optional field - servers without it continue to work
• Type-safe deserialization using existing converter patterns

[blkt]

Looks good to me!