kustomize generator: add ignore file

[cbarbian-sap]

This PR add the possibility to make the KustomizeGenerator ignore certain files by creating a file called .component-ignore in the kustomization directory (that is the directory passed to NewKustomizeGenerator().
The file uses the usual .gitignore syntax.

Notes:

• Other than git we honour this file only in the root of the specified kustomization, but not in any subdirectories.
• The files .component-config.yaml and .component-ignore are always included.
• Files excluded will not be rendered, and will not be included into a generated kustomization.yaml (in case the directory does not have one).
• However, excluded files can still be retrieved by the readFile function (and related functions).