[mdatagen] Add a generated test calling NewFactory().createDefaultConfig().Validate()

[atoulme]

This test should ensure the following:

• We can call the Validate() function in an idempotent manner (meaning, the Config object is not modified by a call to Validate)
• The Validate() call does not depend on external resources such as network calls.

[krisztianfekete]

Trying to wrap my head around mdatagen conventions, let me know if my thinking makes sense:

• This is about adding the idempotency test in cmd/mdatagen/internal/templates/component_test.go.tmpl, so it can be re-used across components since they implement Config interface with Validate()
• The test should call Validate() multiple times, comparing results/warnings, etc. to see whether those has changed or not, this should be straightforward
• Re. external dependencies: do you have any specific approach in mind?

[atoulme]

No specific approach, but any call out to an authenticated resource such as aws ec2 metadata services or an environment variable will fail (though, I did add env var support for tests).