Passing context.Context to Helper methods

[matejvasek]

Passing context.Context to the helper methods would make them cancellable.

type CtxAwareHelper interface {
	// Add appends credentials to the store.
	Add(context.Context, *Credentials) error
	// Delete removes credentials from the store.
	Delete(context.Context, serverURL string) error
	// Get retrieves credentials from the store.
	// It returns username and secret as strings.
	Get(context.Context, serverURL string) (string, string, error)
	// List returns the stored serverURLs and their associated usernames.
	List(context.Context) (map[string]string, error)
}

Some helper implementation like aws ecr may hang for a long time on some network timeouts. It would be neat we could limit that via some context.