Ability to customize DefaultImagePrefix at runtime

[mozesl-nokia]

Currently, when running the kpt CLI, the default image prefix is hardcoded to be the GHCR prefix.

This should be customizable with and environment variable, a CLI argument or both.

[kavirakesh14]

Hi @mozesl-nokia , I'd like to work on this issue, please assign this to me and i'll investigate and raise a PR. Thanks