Make version on element level optional (already there for local, required for external resources)

[morri-son]

What would you like to be added:
Although specified asoptional in the OCM Specification for the element identity, it is required to specify a version on resource level when not describing local resources, but an access to an external resource. For local resources it can be omitted and gets implicitly the version defined on component level.

The format for the version name is semver.

It should not be required to set a version on element level, regardless if for a local resource of for one described by an external access.
The version format should not be restricted, because the structure of the version value has no functional meaning for the model.

Why is this needed:
It is a bit confusing why the version is required for a subset for resources in the component constructor. The format should be user specific, to be able to provide technology- or process-specific values describing the version from the origin of the artifact. (The version of the component version must be semver)