Make Product Registry configurable

[clausa]

Please check below, if the PR fulfills these requirements:

• The commit message follows the guidelines.
  Hopefully - This is my first PR...
• Tests for the changes have been added (for bug fixes / features).
• Docs have been added / updated (for bug fixes / features).

What kind of change does this PR introduce?

• [RFE] Configurable Product Registry #193

What is the current behavior?

• Product Registry is hardcoded to rgcrprod.azurecr.us

What is the new behavior (if this is a feature change)?

• You can specify a Product Registry with the --product-registry (or -c ?) cli flag.

Does this PR introduce a breaking change?

• No

Other information:

bin/hauler store sync --products rke2=v1.28.5+rke2r2 --product-registry mcr.csa-net.dk:5000  
10:12PM INF processing content file for product: 'rke2=v1.28.5+rke2r2'
10:12PM INF retrieving product manifest from: 'mcr.csa-net.dk:5000/hauler/rke2-manifest.yaml:v1.28.5-rke2r2'
10:12PM INF added 'image' to store at [mcr.csa-net.dk:5000/hauler/rke2-manifest.yaml:v1.28.5-rke2r2]
10:12PM INF extracted [application/vnd.oci.image.manifest.v1+json] from store with digest [sha256:6b811e04ced86b53d5f76f8a043332325c2bb440238587266b2b27a10187a97f]
10:12PM INF extracted [application/vnd.oci.image.manifest.v1+json] from store with digest [sha256:e86ba80365854ce14a0f7e9b91b46bd105a2220901fcbb2fde08ebbe9e03444c]
10:12PM INF syncing [content.hauler.cattle.io/v1alpha1, Kind=Images] to store
10:12PM INF added 'image' to store at [mcr.csa-net.dk:5000/docker.io/rancher/hardened-calico:v3.26.3-build20231109]

[dweomer]

Thank you @clausa for this contribution! We are amenable to this change but want to hold back a week or two to make a design pass at a roadmap to ensure this fits.