Add flag to allow re-using of existing namespace

[AverageMarcus]

Currently it's not possible to use an existing namespace for hydrophone as it prompts you to run a cleanup if the provided namespace already exists.

It would be nice to have a flag similar to Sonobuoys --skip-preflight=existingnamespace that can instruct Hydrophone to use an existing namespace.

The reason for this is we generally run clusters with strict network policies enforced by default that prevents cross-namespace communication. To still allow the conformance tests to run and be triggered we need to create a CiliumNetworkPolicy within the conformance namespace but cannot do that before running hydrophone currently.