Allow kustomizations to include files/kustomizations outside their path

[cbarbian-sap]

So far, instances of the KustomizeGenerator are 'fenced' to the directory which is passed as kustomizationPath when instantiating it. That means:

• their kustomization.yaml (if present) cannot reference resources (bases) or patches outside that directory, such as ../bases
• the readFile template function cannot reference files outside kustomizationPath.

This gap is addressed through this PR: two new attributes are recognized in .component-config.yaml:

• includedKustomizations: []string: a list of directory paths relative to kustomizationPath; targeted directories are treated as own components, rendered with the including component's parameters (values), and then supplied to kustomize at the identical path
• includedFiles: []string: a list of paths relative to kustomizationPath (single files or directories); all referenced files (recursively in case a directory is specified) can be used with readFile.

Recursive inclusions are possible, but must not lead to cycles (there is a circuit breaking logic that will fail the generator in case of cycles).

Example:

# mycomp/.component-config.yaml
includedFiles:
- ../meta.yaml
includedKustomizations:
- ../bases

Then, mycomp/kustomization.yaml can reference ../bases, such as

# mycomp/kustomization.yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- ../bases
# ...

and templates in mycomp can call readFile "../meta.yaml".