add new manifest deployer update strategies

[reshnm]

How to categorize this PR?

/area manifest-deployer
/kind enhancement
/priority 3

What this PR does / why we need it:

The previously available manifest deployer update strategies update and patch were not sufficient for resources that are being modified by a kubernetes controller after being created (like Gardener Shoots). The available strategies would always overwrite these modfications.

This PR introduces two new update strategies to accomodate the use case scenarios:

• merge: The manifest deployer will merge the results of the rendered manifests into the resources on the cluster. Fields that already exist in the resources on the cluster, will not be overwritten.
• mergeOverwrite: The manifest deployer will merge the results of the rendered manifests into the resources on the cluster. Fields that already exist in the resources on the cluster, will be overwritten when the rendered field is not empty.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Add manifest deployer update strategies "merge" and "mergeOverwrite".

[Diaphteiros]

/lgtm

[reshnm]

/test

[testmachinery]

Testrun: e2e-28frm
Workflow: e2e-28frm-wf
Phase: Failed

+--------------------+--------------------+-----------+----------+
|        NAME        |        STEP        |   PHASE   | DURATION |
+--------------------+--------------------+-----------+----------+
| create-cluster     | create-cluster     | Succeeded | 3m13s    |
| create-registry    | create-registry    | Succeeded | 2m46s    |
| install-landscaper | install-landscaper | Succeeded | 53s      |
| integration-test   | test               | Failed    | 6m15s    |
| delete-cluster     | delete-cluster     | Omitted   | 0s       |
| delete-registry    | delete-registry    | Omitted   | 0s       |
+--------------------+--------------------+-----------+----------+