Create OCM context only if necessary

[robertgraeff]

What this PR does / why we need it:

Deployers have always created an OCM Context at the beginning of the Reconcile method. As a consequence, OCM Contexts were often created unnecessarily, for example if the deployer was not responsible for a DeployItem, or if the DeployItem was already in a finished phase. With this PR the creation of OCM Contexts is postponed, and therefore skipped if not needed.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

- Reduction of memory consumption by creating OCM Contexts only if needed.

[gardener-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all