[FEATURE] MCP Servers configuration : add expanding variables support

In order for the project scope mcp servers to really be useful, it would be great that the config allowed for expanding environment variables.

For instance in a project .mcp.json :

{
  "mcpServers": {
    "mysql_local": {
      "type": "stdio",
      "command": "uvx",
      "args": [
        "--with",
        "pymysql",
        "mcp-alchemy"
      ],
      "env": {
        "DB_URL": "mysql+pymysql://$USER@localhost:3306/database"
      }
    }
  }
}

The $USER variable isn't expanding, forcing everyone to install this mcp server locally and rendering it useless as a project mcp.