Add support for resource link

[ihrpr]

Implements support for ResourceLinks
Closes #618

What are ResourceLinks?

ResourceLinks allow tools to return references to resources without embedding their full content. This is essential for performance when dealing with large files or many resources - clients can then selectively read only the resources they need.

Implementation

• Added ResourceLinkSchema type that extends ResourceSchema with type: "resource_link"
• Created ContentBlockSchema union type that includes all content types (text, image, audio, resource_link, resource)
• Updated PromptMessageSchema and CallToolResultSchema to use the new ContentBlockSchema
• Added type exports for ResourceLink and ContentBlock

Example Usage

  // Tool returning ResourceLinks
  server.registerTool('list-files', {
    inputSchema: { pattern: z.string() }
  }, async ({ pattern }) => ({
    content: [
      { type: 'text', text: `Found files:` },
      {
        type: 'resource_link',
        uri: 'file:///project/README.md',
        name: 'README.md',
        mimeType: 'text/markdown'
      }
    ]
  }));

[bhosmer-ant]

LGTM!