Delete the DNSEndpoint resource when VS is deleted & Ratelimit requeues on errors.

[ciarams87]

Proposed changes

When a VirtualServer with ExternalDNS enabled is deleted, the corresponding DNSEndpoint resource is not cleaned up. Additionally, we are endlessly re-queuing VS objects with ExternalDNS enabled with errors without rate limiting.
Also, we are not checking if the VS has been deleted before re-queuing, in both the external dns controller, and the cert-manager controller.

This commit rectifies these issues by:

1. Not blocking owner deletion on DNSEndpoints
2. When there is an error processing a DNSEndpoint sync for VirtualServer, the VS is re-added to the queue using rate limiting, with an eventual timeout
3. When the error processing the item is due to the VS no longer existing, don't return an error and exit quietly.

Checklist

Closes #4418

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (c7680a4) 51.99% compared to head (5a8ddf2) 51.94%.

Files	Patch %	Lines
internal/externaldns/controller.go	0.00%	6 Missing ⚠️
internal/certmanager/cm_controller.go	0.00%	4 Missing ⚠️
internal/externaldns/sync.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4504      +/-   ##
==========================================
- Coverage   51.99%   51.94%   -0.05%     
==========================================
  Files          59       59              
  Lines       16960    16972      +12     
==========================================
- Hits         8818     8816       -2     
- Misses       7847     7859      +12     
- Partials      295      297       +2     

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