[gardenlet] Switch extensions controller to controller-runtime

[rfranzke]

How to categorize this PR?

/area dev-productivity scalability
/kind enhancement

What this PR does / why we need it:
Refactor the extensions controller to controller-runtime. It consists of two reconcilers, so there are some empty separator commits to allow easy navigation through the PR.

Instead of keeping the old structure, it is now as follows:

• The "ControllerInstallation Required" reconciler was moved to pkg/gardenlet/controller/controllerinstallation/required
• The "ShootState Extensions Sync" reconciler was moved to pkg/gardenlet/controller/shootstate/extensions

Which issue(s) this PR fixes:
Part of #4251

Special notes for your reviewer:
Generally, we want to follow this cookbook while refactoring existing controllers:

1. Add documentation
2. Add integration test based on envtest (if not already present)
3. Switch controller to controller-runtime

Release note:

NONE

[rfranzke]

• The "ShootState Extensions Sync" reconciler was moved to pkg/gardenlet/controller/shootstate/extensions

cc @acumino for the refactoring of the shootsecret controller: Please keep this structure, i.e. move it to pkg/gardenlet/controller/shootstate/secret.

[gardener-prow]

@rfranzke: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-dependency-watchdog-verify-image-build	b34ccc2	link	true	/test pull-dependency-watchdog-verify-image-build
pull-gardener-check-vulnerabilities	845523f	link	false	/test pull-gardener-check-vulnerabilities

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see our testing guideline for how to avoid and hunt flakes.

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[rfranzke]

/approve

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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