read soft_delete from provider before acting

In order to avoid race conditions on soft_delete which could lead to an NXDomain response we should first write the soft_delete label to the DNS Provider as a way of signalling and intention to delete.

The delete can then be performed once read back from the provider and assuring that unlabelled endpoints still remain.

Note that this requires a change to the TXT registry to function: https://github.com/Kuadrant/dns-operator/issues/346

Start by reading the DNS provider interaction and the TXT registry, then review the related change in issue #346. The intended flow is to write the soft_delete label, read it back from the provider, and delete only after confirming unlabelled endpoints remain; done includes the required TXT registry change.

Written by the indexing model from the issue text.