feat: add support for targets to cargo

Add support for targets in buildpack.toml which
are used for multi-arch support.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

cargo/config.go

@@ -15,13 +15,19 @@ type Config struct {
 	Metadata  ConfigMetadata  `toml:"metadata"  json:"metadata,omitempty"`
 	Stacks    []ConfigStack   `toml:"stacks"    json:"stacks,omitempty"`
 	Order     []ConfigOrder   `toml:"order"     json:"order,omitempty"`
+	Targets   []ConfigTarget  `toml:"targets"   json:"targets,omitempty"`
 }
 
 type ConfigStack struct {
 	ID     string   `toml:"id"     json:"id,omitempty"`
 	Mixins []string `toml:"mixins" json:"mixins,omitempty"`
 }
 
+type ConfigTarget struct {
+	OS     string   `toml:"os"     json:"os,omitempty"`
+	Arch   string   `toml:"arch"   json:"arch,omitempty"`
+}
+
 type ConfigBuildpack struct {
 	ID          string                   `toml:"id"                    json:"id,omitempty"`
 	Name        string                   `toml:"name"                  json:"name,omitempty"`

cargo/config_test.go

@@ -57,6 +57,12 @@ func testConfig(t *testing.T, context spec.G, it spec.S) {
 						ID: "other-stack-id",
 					},
 				},
+				Targets: []cargo.ConfigTarget{
+					{
+						OS:     "linux",
+						Arch:   "arm64",
+					},
+				},
 				Metadata: cargo.ConfigMetadata{
 					IncludeFiles: []string{
 						"some-include-file",
@@ -166,6 +172,10 @@ api = "0.6"
 [[stacks]]
   id = "other-stack-id"
 
+[[targets]]
+  os = "linux"
+  arch = "arm64"
+
 [[order]]
   [[order.group]]
 	  id = "some-dependency"