Platforms: Add From/ToProto helpers for types

Helpers to convert from a slice of platforms to our protobuf representation
and vice-versa appear a couple times. It seems sane to just expose this facility
in the platforms pkg.

Signed-off-by: Danny Canter <danny@dcantah.dev>

platforms/platforms.go

@@ -116,6 +116,7 @@ import (
 
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 
+	"github.com/containerd/containerd/api/types"
 	"github.com/containerd/containerd/errdefs"
 )
 
@@ -262,3 +263,30 @@ func Normalize(platform specs.Platform) specs.Platform {
 
 	return platform
 }
+
+// ToProto converts from a slice of [Platform] to a slice of
+// the protobuf definition [types.Platform].
+func ToProto(platforms []Platform) []*types.Platform {
+	ap := make([]*types.Platform, len(platforms))
+	for i := range platforms {
+		p := types.Platform{
+			OS:           platforms[i].OS,
+			Architecture: platforms[i].Architecture,
+			Variant:      platforms[i].Variant,
+		}
+		ap[i] = &p
+	}
+	return ap
+}
+
+// FromProto converts a slice of the protobuf definition [types.Platform]
+// to a slice of [Platform].
+func FromProto(platforms []*types.Platform) []Platform {
+	op := make([]Platform, len(platforms))
+	for i := range platforms {
+		op[i].OS = platforms[i].OS
+		op[i].Architecture = platforms[i].Architecture
+		op[i].Variant = platforms[i].Variant
+	}
+	return op
+}