Additional targets

[robertgraeff]

What this PR does / why we need it:

This PR adds support for:

• a secondary target cluster in custom readiness checks, export collection, and deletion groups,
• patching resources after their deployment, resp. before their deletion.

The first feature can be used in scenarios where the Landscaper deploys an "installer" on a target cluster 1, and then this installer creates further resources on a target cluster 2. In such a case, a readiness check typically inspects resources on target cluster 2.

The second feature allows for example to suspend an installer when a deployment has been finished.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

- Support for a secondary target cluster in custom readiness checks, export collection, and deletion groups.
- Support for patching resources after their deployment, resp. before their deletion.

[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