add creationTime attribute

[mandelsoft]

What this PR does / why we need it:

It adds an optional creationTimestamp field to the object meta information.

It uses a format according to RFC3339.

The creation time will be set by the CLI commands creating component versions.
For a library user, the timestamp has to explicitly be added to the component descriptor when
creating a new version.

Correlates to ocm-spec PR#49.

Which issue(s) this PR fixes:
F[ixes #48

Special notes for your reviewer:

Release note:

[In-Ko]

• Proper documentation of the new element CreationDate is missing (in the OCM and OCM-Spec docs)
• Only one JSON-Schema has been adjusted (the one in the OCM Repo). But there is another one in the OCM-Spec Repo.
  • The OCM-Spec Schema also needs to be changed
  • This begs the question: Why do we have two separate JSON-Schema anyway ? Is that really necessary ? (If we want do change this, we should do this in another PR though ...)

[mandelsoft]

Aligned spec with PR#49

[In-Ko]

• Discussion about only having 1 JSON Schema is ongoing, will be tackled in a different work package / task.
• With PR Add specification for provider an creationTime field ocm-spec#49, the JSON Schema will be updated, so now this PR LGTM.

[In-Ko]

LGTM