New source type httpRepository

[cbarbian-sap]

This PR adds a new source type httpRepository. It allows to reference the source manifests as a gzipped tar archive on an arbitrary http location. Such as

apiVersion: core.cs.sap.com/v1alpha1
kind: Component
metadata:
  name: test
spec:
  sourceRef:
    httpRepository:
      url: http://localhost:8888/sources/test/latest.tar.gz

The response received from the given URL must specify a content digest through a certain header field. By default, the ETag header is used for this, but a different header name can be configured by setting spec.sourceRef.httpRepository.revisionHeader. The value of that header must uniquely identify the returned content (e.g. be a hash of the returned archive). Redirects will be followed as long as the redirect response does not contain the revision header field.

Note that the operator will not only make GET but also HEAD requests to the given URL (and potentially returned redirect locations). The target http server therefore must support HEAD requests.