Add API examples (#2517)

* [wip] initial syft api examples

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* smooth over some rough edges in the API

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* embed example file

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* address review comments

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* change name of builder function

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

---------

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

examples/decode_sbom/main.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	_ "embed"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/anchore/syft/syft/format"
+)
+
+//go:embed alpine.syft.json
+var sbomContents string
+
+func main() {
+	// decode the SBOM
+	fmt.Println("decoding SBOM...")
+	sbom, sbomFormat, formatVersion, err := format.Decode(sbomReader())
+	if err != nil {
+		fmt.Printf("failed to decode sbom: %+v\n", err)
+		os.Exit(1)
+	}
+
+	fmt.Printf("SBOM format: %s@%s\n", sbomFormat, formatVersion)
+
+	// print packages found...
+	fmt.Println("\nPackages found:")
+	for _, pkg := range sbom.Artifacts.Packages.Sorted() {
+		fmt.Printf("   %s : %s@%s (%s)\n", pkg.ID(), pkg.Name, pkg.Version, pkg.Type)
+	}
+
+	// print files found...
+	fmt.Println("\nFiles found:")
+	for c, f := range sbom.Artifacts.FileMetadata {
+		fmt.Printf("   %s : %s\n", c.ID(), f.Path)
+	}
+}
+
+func sbomReader() io.Reader {
+	// read file from sys args (or use the default)
+	var reader io.Reader
+	if len(os.Args) < 2 {
+		reader = strings.NewReader(sbomContents)
+	} else {
+		var err error
+		reader, err = os.Open(os.Args[1])
+		if err != nil {
+			panic(err)
+		}
+	}
+	return reader
+}