Provide an option to istio-must-gather to scrape only selected SMCP

We have a diagnostic tool for capturing Service Mesh-specific resources and logs from a cluster - istio-must-gather.

Currently it scrapes all existing Service Mesh Control Plane instances with all dependant objects.

This brings a problem with large multi-tenant deployments. If a customer has 10-20 SMCPs and has issues to troubleshoot only in one of them - istio-must-gather will capture the whole setup.

This takes huge amount of time and consumes a lot of storage - and this in turn makes much harder to move from disconnected cluster to support case.

Also, analyzing of such big tarball is difficult and tends to human errors: it's necessary to find a specific namespace there and not to mix it with healthy ones.

Proposed feature is to add to istio-must-gather command an option to specify a SMCP namespace to scrape, so limiting a volume of collecting data and easy its further analysis.

If command-line option can't be used for istio-must-gather due to 'oc' cli tool limitations, we can think of labeling necessary SMCP namespace by predefined label or move this requested functionality to future OSSM-specific cli tooling (OSSM-5 RFE).