Finish REST driver implementation (some pieces were left out from initial implementation)

[mesembria]

[teodor-yanev]

I need to talk with @JAORMX before picking this up

[teodor-yanev]

Looking at the TODOs in internal/datasources/rest/handler.go, I do not see any of them being critical for releasing data sources in prod for the time being. They include:

• implement fallback
• implement auth (we already discussed that this is not needed for now as we only hit public endpoints in our rules for the time being)
• add an option to use custom clients and make the timeout configurable (I'm afraid I miss the context here)

cc: @eleftherias @evankanderson

[eleftherias]

I agree that none of those are critical. We can pick them up at a later time.

[evankanderson]

Does this mean that we should enable the datasources feature flag in production?