v1.63.0: Improve ReadBuildpackYaml

ReadBuildpackYaml now receives an interface and allows unmarshalling of arbitrary configuration structures

• This allows CNBs to define configuration they care about and tell the helper to unmarshal it for the,.

# CNB Define this structure including top level key at which to find their configuration
type BuildpackYaml struct {
		Python struct {
			Version string `yaml:"version"`
		} `yaml:"python"`
	}

(it also streams during the unmarshal)