Add define_custom_method to define custom methods (and notifications)

[atesgoral]

Closes #2

Adds define_custom_method to Server to allow definition of custom methods (outside the MCP spec). Method blocks that return nil become custom notifications.

Motivation and Context

MCP supports both servers and clients to send notifications. And notifications are not limited to what's defined by the spec.

This PR adds flexibility for servers to receive custom method calls and notifications from clients/hosts.

Servers can implement cutting-edge MCP methods before this gem implements them.

Custom notifications from host to server allow use cases like diagnostic logging, capturing of multimodal LLM transcripts, etc. by piggybacking on MCP instead of implementing custom endpoints.

How Has This Been Tested?

New unit tests

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[koic]

If I were to raise one minor concern, it would be whether supporting methods outside the MCP specification is entirely suitable. That said, based on the explanation in the PR, it seems there’s a clear rationale for needing such functionality.

Both the documentation and the implementation look solid 👍