Add grafana_data_source_config resource

[miljanic]

This PR introduces the grafana_data_source_config resource. It will separate the creation of the Data Source from its configuration. This is especially useful in cases where multiple Data Sources configurations depend on each other, thus creating a circular dependency.

Closes #1408

[CLAassistant]

All committers have signed the CLA.

[github-actions]

In order to lower resource usage and have a faster runtime, PRs will not run Cloud tests automatically.
To do so, a Grafana Labs employee must trigger the cloud acceptance tests workflow manually.

[julienduchesne]

Nice, thanks for picking this up. Some things need fixing but this is a good start

[miljanic]

@julienduchesne I fixed all comments but the one related to the common code extraction. I understand its idea, but couldn't find an idiomatic way to do so. I'm still happy to do it if you give me an example.

[julienduchesne]

@julienduchesne I fixed all comments but the one related to the common code extraction. I understand its idea, but couldn't find an idiomatic way to do so. I'm still happy to do it if you give me an example.

I can take a look 👀

[julienduchesne]

Added a test, fixed up a few things and make a bunch of code common between the regular datasource resource. LGTM now! Thank you!