Signing with multiple scheme versions

[mandelsoft]

What would you like to be added:

The current signing solution works only with a single persistence version for the component descriptor.
But we will for sure have at least one more. All desired changes have been postponed so far and will lead to a new more
K8s like representation.

The new implementation of the component lib already introduced an internal version, which is used by the library function,
only for the external representation a dedicated schema version is generated.

Therefore we have to think about the way we handle signatures, again:

• a signature is scheme specific. In this case it would be a good idea to add the scheme a signature is intended for to the
  signature/digest spec to be able to store information for multiple scheme versions. This would be useful to be able to transform the
  external representation without loosing the signature.
• a signature is built on the internal version, which would work if all internal versions can express the same state. If the state is
  extended, a signature will again be valid only for a dedicated subset of the features of the complete descriptor. This again leads to
  the necessity to add information to the signature/digest spec.

Why is this needed: