How can I create a signed a URL to invoke a lambda?

I want to have a signed URL that an external service can call with either {"success": true} or {"success": false} in the payload. I could create an endpoint to do this, but that requires a bunch of extra work and adds more security issues to have to think about. I see docs around sigv4, and how to create your own signed request, but I don't see any APIs that handle this for you. Is there any way to do this without rolling my own request signer?