Allow Custom Client Features for MCP Tools

MCP tools can only utilize the predefined Client Roots and Sampling client features, which are quite restrictive. Tools often require dynamic information from the client/host, but the MCP currently does not facilitate this.

We can address this limitation by allowing the definition of custom client features.

For example, if the client defines a custom feature called get/context that returns some dynamic context information needed by the get-context-from-client-and-do-something tool:

server.tool(
  'get-context-from-client-and-do-something',
  async (_, { sendRequest }) => {
    const context = await sendRequest({ method: 'get/context', params }, resultSchema);
  
    // Do something with the context. 
    
    return {
      content: [
        {
          type: 'text',
          text: `${result}`,
        },
      ],
    };
);

Start by tracing the client feature handling for the predefined Roots and Sampling features, then inspect the server.tool callback and its sendRequest entry point. Compare the current request flow with the custom get/context example. Done means a client can define a custom feature and an MCP tool can request it with the expected result handling.