oc-mirror support exporting metadata from registry-based storageConfig

1. Proposed title of this feature request
export the oc-mirror metadata from local registry when the metadata is stored in registry-based storageConfig

2. What is the nature and description of the request?
Currently, the oc-mirror describe command can only export the metadata from specific tar archive file, but since the tar file may not represent the single source of truth about the latest metadata information it is not convenient to display  the up-to-date metadata info. The command does not support dump the metadata when it is stored in the registry.  

We need to enhance the "describe" subcommand to let it support displaying the metata from registry, such as:

$ oc-mirror describe https://registry.example.com:5000/v2/mirror/oc-mirror-metadata 

we may also need to consider the authentication, skipTLS, etc.

3. Why does the customer need this? (List the business requirements here)

Customer need this feature to view the metadata for backup purpose, comparison with another metadata to display the diff or troubleshooting purpose

4. List any affected packages or components.
oc-mirror