Improve testing for Runtime SDK variable discovery

[killianmuldoon]

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.

[killianmuldoon]

/area runtime-sdk
/assign

[killianmuldoon]

/triage accepted

[killianmuldoon]

/unassign
/help

[k8s-ci-robot]

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[fabriziopandini]

/kind cleanup
/priority important-longterm