This introduces the `mcp-server` crate, which contains a barebones MCP server that provides an `echo` tool that echoes the user's request back to them. To test it out, I launched [modelcontextprotocol/inspector](https://github.com/modelcontextprotocol/inspector) like so: ``` mcp-server$ npx @modelcontextprotocol/inspector cargo run -- ``` and opened up `http://127.0.0.1:6274` in my browser:  I also had to make a small fix to `mcp-types`, adding `#[serde(untagged)]` to a number of `enum`s.