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

## 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](https://pkg.go.dev/github.com/google/go-containerregistry/pkg/name) documentation

Similar defaulting logic in [name.go](https://github.com/google/go-containerregistry/blob/main/pkg/name/name.go)