feat: ClientGroup for managing multiple MCP server connections

[amirh]

Motivation and Context

This stack of changes makes it easier to work with multiple MCP servers from a single client.
This has also been requested in modelcontextprotocol#6

To make the review easier with smaller focused PRs I'm sending a stack or PRs with incremental implementation steps.

This PR is adding the initial ClientGroup class for managing a group of Clients.

Currently ClientGroup allows for convenient listing and invocations of tools from across multiple clients, and convenience for closing all clients.

Still missing:

• Proper way for handling tool name conflicts.
• Support for prompts and resources (I want to get the API and semantics around tools approved first).

PR Stack

• sample: Add a sample using a group of MCP clients. modelcontextprotocol/typescript-sdk#486
• feat: ClientGroup for managing multiple MCP server connections #1
• feat: Allow renaming tools in ClientGroup #2

Note that since I'm sending the PRs from a fork, only the PR at the bottom of the stack is sent to modelcontextprotocol/typescript-sdk. As PRs land I'll update the target repository for the new base PR.

How Has This Been Tested?

• I added unit tests and they are passing.
• I ran: npm run build && node dist/esm/examples/client/clientGroupSample.js

Breaking Changes

No breaking changes.

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