Task: Add _meta Field to Interface Types

[ezynda3]

Priority: Low
Breaking Change: No

Description: Additional interface types now support the _meta field for extensibility.

Implementation Tasks:

• Add _meta field to Resource, Tool, Prompt, and other interface types
• Update JSON marshaling to handle _meta properly
• Add documentation about _meta usage
• Ensure _meta is reserved for protocol use

Files to Update:

• mcp/types.go
• All struct definitions that should support _meta