Improve fetching troubleshoot specs from OCI registries

[banjoh]

Describe the rationale for the suggested change.

Looking at the implementation of pulling specs from OCI registries, its not clear how to use this API and in extension, the CLI. The issues I see are

• The media types and image names are hard coded meaning the library can only work with 2 image names
• The oci URL format is not clear. This is more a documentation bit than an implementation one. Usage documentation may lead to code change
• The oras library is old and needs upgrading -> #1317
• There is a hard coded helm credentials path. The larger deficiency here is the implementation of authenticating to registries

Description of tasks

• Refactor this code
  • Some work has been done as part of #1337