Enable CLA for this project

[brylie]

In order to hold the Copyright to all contributions to this codebase, APInf needs developers to sign a Contributor License Agreement (CLA).

Goal

Require contributors to sign a CLA before accepting contributions to this project. The CLA process should be as simple as possible (read: low friction).

Resources

CLAHub offers an automated CLA checking service for Github projects.

[tulir]

We should wait for apinf/openapi-designer#200 (which is waiting for apinf/platform#2690) and then use the same CLA as that.