Custom grafana datasource plugin to wrap external REST API

You can use Grafana Infinity datasource for connecting to REST API endpoints.

This plugin can connect to any REST API and present the data to grafana. You can use CSV,JSON and GraphQL endpoints. Documentation site have more examples on how to connect with external APIs

Note: I am the author of the plugin. If you see any bugs in the datasource, report it at github site.

The Simple JSON Datasource does roughly what you're proposing, and would definitely be a good base for you to start from.

There is also documentation on datasource plugins available.