Allow oci:// protocol to be specified for OCI AppRepositories

[absoludity]

Description of the change

When OCI support was added originally, we required people to specify an http/https URL for the registry, even though Helm itself allows oci://, because we're (Kubeapps) actually interacting with the OCI distribution spec in that code.

In addition, there was an issue in the dashboard that even if you did specify oci://registry-1.docker.io/foo the dashboard would assume the URI was missing a protocol since it doesn't start with http and add it, resulting in https://oci://registry-1.docker.io/foo.

This PR fixes the dashboard issue (so you can specify an oci protocol), updates the validation in the kubeapps-apis to use https in that case, and the sync code to do the same.

• TODO: check that the oci catalog handles it as well. (all good - the grpc API only takes the host and namespace etc.

Benefits

Users can specify the OCI registry the way they would with Helm or other tools (this actually tripped up someone trying it out recently).

Possible drawbacks

Bugs?

Applicable issues

• fixes Support oci:// as the protocol when adding an OCI registry/namespace source #6427

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[antgamdia]

Thanks for the fix!

I just remembered we had an open issue partially related to this code (the repo validation, in general). I'm referencing it just in case we wanted to have a look at: #5781