bake: initial set of composable bake attributes

This allows using either the csv syntax or object syntax to specify
certain attributes.

This applies to the following fields:
- output
- cache-from
- cache-to
- secret
- ssh

There are still some remaining fields to translate. Specifically
ulimits, annotations, and attest.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

bake/hclparser/gohcl/encode_test.go

@@ -0,0 +1,67 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package gohcl_test
+
+import (
+	"fmt"
+
+	"github.com/hashicorp/hcl/v2/gohcl"
+	"github.com/hashicorp/hcl/v2/hclwrite"
+)
+
+func ExampleEncodeIntoBody() {
+	type Service struct {
+		Name string   `hcl:"name,label"`
+		Exe  []string `hcl:"executable"`
+	}
+	type Constraints struct {
+		OS   string `hcl:"os"`
+		Arch string `hcl:"arch"`
+	}
+	type App struct {
+		Name        string       `hcl:"name"`
+		Desc        string       `hcl:"description"`
+		Constraints *Constraints `hcl:"constraints,block"`
+		Services    []Service    `hcl:"service,block"`
+	}
+
+	app := App{
+		Name: "awesome-app",
+		Desc: "Such an awesome application",
+		Constraints: &Constraints{
+			OS:   "linux",
+			Arch: "amd64",
+		},
+		Services: []Service{
+			{
+				Name: "web",
+				Exe:  []string{"./web", "--listen=:8080"},
+			},
+			{
+				Name: "worker",
+				Exe:  []string{"./worker"},
+			},
+		},
+	}
+
+	f := hclwrite.NewEmptyFile()
+	gohcl.EncodeIntoBody(&app, f.Body())
+	fmt.Printf("%s", f.Bytes())
+
+	// Output:
+	// name        = "awesome-app"
+	// description = "Such an awesome application"
+	//
+	// constraints {
+	//   os   = "linux"
+	//   arch = "amd64"
+	// }
+	//
+	// service "web" {
+	//   executable = ["./web", "--listen=:8080"]
+	// }
+	// service "worker" {
+	//   executable = ["./worker"]
+	// }
+}