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>

platforms/defaults.go

@@ -22,8 +22,13 @@ import (
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
-// Default returns the default specifier for the platform.
-func Default() string {
+// Default returns the default matcher for the platform.
+func Default() MatchComparer {
+	return Only(DefaultSpec())
+}
+
+// DefaultString returns the default string specifier for the platform.
+func DefaultString() string {
 	return Format(DefaultSpec())
 }