Improve testing for Runtime SDK variable discovery

This is an umbrella issue to follow up on improvements to testing and documenting #7985

• Add unit tests testing the variables returned from the test-extension
• Add an end to end test that covers complex variables
• Add an end to end test which covers moving from a set of inline variables and patches to external variables and patches
• Use runtimeSDK settings in a runtime variable discovery and/or patch call
• Add documentation on best practices for writing variable discovery extensions.