Allow overriding the default repository prefix when parsing image references in `pkg/name`

[ilopezluna]

Description

The pkg/name package currently interprets shorthand image names like "ubuntu" as
"index.docker.io/library/ubuntu:latest", automatically defaulting missing fields to:

• Registry: index.docker.io
• Repository prefix: library
• Tag: latest

While convenient, this behavior cannot currently be customized. It would be useful to support environments where the default repository prefix differs from "library" (e.g., "default" or "base").

Proposed solution

Add an optional argument or configuration option to let users specify the default repository prefix when parsing references, for example:

ref, err := name.ParseReference("ubuntu", name.WithDefaultRepo("default"))
// -> index.docker.io/default/ubuntu:latest

or via a package-level configuration:

name.SetDefaultRepo("default")

This would maintain backward compatibility (still defaulting to "library") while allowing users to adapt the behavior to custom registries or mirrors.

Motivation

This feature would help:

• Integrate with registries that use different default namespaces.
• Improve flexibility when working in registry setups.
• Maintain consistency with existing configuration patterns in the package (e.g. WithDefaultRegistry).

References

pkg/name documentation

Similar defaulting logic in name.go