Proposal to add custom must-gather image option in MusTtGather Spec.

[shivprakashmuley]

This PR introduces an enhancement proposal to support the use of custom must-gather images with the must-gather-operator. This feature allows cluster administrators to define an allowlist of trusted images that can be used for diagnostic data collection, providing a secure and flexible way to gather specialized information.

Summary:

The proposal introduces two main API changes:
A new MustGatherImage CRD: This cluster-scoped resource acts as an allowlist for custom must-gather images. Cluster administrators can manage this resource to control which images are permitted to run in the cluster.
An update to the MustGather CRD: The MustGather CRD is extended with an optional mustGatherImage field. When creating a MustGather resource, users can specify an image from the allowlist to be used for the data collection job.
The must-gather-operator's logic is updated to validate the mustGatherImage against the MustGatherImage allowlist. If the image is valid, the operator will use it to run the must-gather job. If the image is not in the allowlist, or if the allowlist is not configured, the MustGather resource's status will be updated with an error. If no custom image is specified, the operator will use the default must-gather image, ensuring backward compatibility.

User-Facing Changes:

Cluster administrators can now create and manage a MustGatherImage resource to control the use of custom must-gather images.
Users can specify a custom must-gather image in the MustGather CR, provided it is in the allowlist.

JIRA tracker:
https://issues.redhat.com/browse/MG-155

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