Add an MCP server to the `bencher` CLI

Add a basic [Model Context Protocol (MCP)](https://modelcontextprotocol.io) server to the `bencher` CLI.

```bash
bencher mcp
```

This would allow for very easy integration:

```json
{
  "mcpServers": {
    "weather": {
      "command": "bencher",
      "args": ["mcp"]
    }
  }
}
```

Rust SDK: https://github.com/modelcontextprotocol/rust-sdk