Add platform match comparer interface

Adds a new platform interface for matching and comparing platforms.
This new interface allows both filtering and ordering of platforms
to support running multiple platform and choosing the best platform.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

cmd/ctr/commands/content/fetch.go

@@ -113,7 +113,7 @@ func NewFetchConfig(ctx context.Context, clicontext *cli.Context) (*FetchConfig,
 	if !clicontext.Bool("all-platforms") {
 		p := clicontext.StringSlice("platform")
 		if len(p) == 0 {
-			p = append(p, platforms.Default())
+			p = append(p, platforms.DefaultString())
 		}
 		config.Platforms = p
 	}