🌱 Add DefinitionFrom field to ClusterVariable

[killianmuldoon]

Add a DefinitionFrom field to the ClusterVariable type for use with conflicting VariableDefinitions.

Part of #7985

[killianmuldoon]

Not sure how much documentation is useful here in terms of the usage of this field. I think it's complex enough that the godoc isn't the right place to describe e.g. that variables can't be used across patches.

[killianmuldoon]

/retest

[sbueringer]

@killianmuldoon
stdout:

The Cluster "k8s-upgrade-with-runtimesdk-ggtj3h" is invalid: 
* spec.topology.variables[0].definitionFrom: Invalid value: "null": spec.topology.variables[0].definitionFrom in body must be of type string: "null"
* spec.topology.variables[1].definitionFrom: Invalid value: "null": spec.topology.variables[1].definitionFrom in body must be of type string: "null"
* spec.topology.variables[2].definitionFrom: Invalid value: "null": spec.topology.variables[2].definitionFrom in body must be of type string: "null"

[sbueringer]

/lgtm

/assign @fabriziopandini

godoc sounds fine to me. I think we could iterate later on if we see in subsequent implementations that we want to add more

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 2297b2fe5225aa37b20ccbc4f5531d8fc430b758

[fabriziopandini]

/lgtm
/approve

[k8s-ci-robot]

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