Schema upgrade mechanism fails to upgrade profiles by default

[corneliusweig]

Currently, the schema upgrade mechanism works by implementing the VersionedConfig interface:

type VersionedConfig interface {
	GetVersion() string
	Upgrade() (VersionedConfig, error)
}

The responsibility of Upgrade is to migrate the whole config from one version to the next. This opens the possibility for a class of bugs, where the main pipeline (SkaffoldConfig.Pipeline) is migrated to a new version, but none of the pipeline configs in the profiles SkaffoldConfig.Profiles[*].Pipeline (see for example #2415). If there are use-cases where profile-pipelines need to be upgraded differently than the main pipeline, the following suggestion is void.

Suggestion

Add a new interface

type SkaffoldPipeline interface {
	Upgrade() (SkaffoldPipeline, error)
}

which upgrades a given pipeline to the next version. In addition, add an upgrade helper which migrates the main pipeline and all profile pipelines to the next version.
This will be backwards compatible with skaffold schema versions before the Pipeline structure was introduced.

[dgageot]

I like that!