feat: supports OAuth (MCP spec: 2025-03-26)

* According to latest spec, you can give url of a remote server that
supports oauth. This means clients can register dynamically with better
security.
(https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization)
* Config like below works:

```json
{
  "my-server": {
    "url": "https://oauth-backed-server/mcp"
  }
}
```

Author: ravitemer