feat: Add MaybeTagStr method to Digest type

[jphelton]

resolves #2069

Details

• Introduces a new method to name.Digest called MaybeTagStr
• The method returns the tag that was included within a digest if it exists

Example Usage

package main

import (
	"fmt"

	"github.com/google/go-containerregistry/pkg/name"
)

func main() {
	i := "gcr.io/project-id/image:1.0@sha256:deadb33fdeadb33fdeadb33fdeadb33fdeadb33fdeadb33fdeadb33fdeadb33f"
	d, _ := name.NewDigest(i)
	tag, ok := d.MaybeTagStr()
	if ok {
		fmt.Printf("Digest %q has a tag: %q", d, tag)
	} else {
		fmt.Printf("Digest %q does not have a tag.", d)
	}

}