feat: add grype db providers command

- currently reads content of `provider-metadata.json` file
- added flag `-o`/`--output` flags which accept `json` and `table`
- update  method `getDBProviders()` and type `dbProviderMetadata` for db schema `v6`

cmd/grype/cli/commands/db.go

@@ -35,6 +35,7 @@ func DB(app clio.Application) *cobra.Command {
 		DBStatus(app),
 		DBUpdate(app),
 		DBSearch(app),
+		DBProviders(app),
 	)
 
 	return db

cmd/grype/cli/commands/db_providers.go

@@ -0,0 +1,138 @@
+package commands
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"strings"
+
+	"github.com/anchore/clio"
+	"github.com/anchore/grype/grype/db/legacy/distribution"
+	"github.com/anchore/grype/internal/bus"
+	"github.com/olekukonko/tablewriter"
+	"github.com/spf13/cobra"
+)
+
+const metadataFileName = "provider-metadata.json"
+
+type dbProviderMetadata struct {
+	Name              string `json:"name"`
+	LastSuccessfulRun string `json:"lastSuccessfulRun"`
+}
+
+type dbProviders struct {
+	Providers []dbProviderMetadata `json:"providers"`
+}
+
+type dbProvidersOptions struct {
+	Output string `yaml:"output" json:"output"`
+}
+
+var _ clio.FlagAdder = (*dbProvidersOptions)(nil)
+
+func (d *dbProvidersOptions) AddFlags(flags clio.FlagSet) {
+	flags.StringVarP(&d.Output, "output", "o", "format to display results (available=[table, json])")
+}
+
+func DBProviders(app clio.Application) *cobra.Command {
+	opts := &dbProvidersOptions{
+		Output: "json",
+	}
+
+	return app.SetupCommand(&cobra.Command{
+		Use:   "providers",
+		Short: "list vulnerability database providers",
+		Args:  cobra.ExactArgs(0),
+		RunE: func(_ *cobra.Command, _ []string) error {
+			return runDBProviders(opts, app)
+		},
+	}, opts)
+}
+
+func runDBProviders(opts *dbProvidersOptions, app clio.Application) error {
+
+	providers, err := getDBProviders(app)
+	if err != nil {
+		return err
+	}
+
+	sb := &strings.Builder{}
+
+	switch opts.Output {
+	case "table":
+		displayDBProvidersTable(providers.Providers, sb)
+	case "json":
+		err = displayDBProvidersJSON(providers, sb)
+		if err != nil {
+			return err
+		}
+	default:
+		return fmt.Errorf("unsupported output format: %s", opts.Output)
+	}
+	bus.Report(sb.String())
+
+	return nil
+}
+
+func getDBProviders(app clio.Application) (*dbProviders, error) {
+
+	dbCurator, err := distribution.NewCurator(dbOptionsDefault(app.ID()).DB.ToCuratorConfig())
+	if err != nil {
+		return nil, err
+	}
+
+	metadataFileLocation := dbCurator.Status().Location
+	metadataFile := path.Join(metadataFileLocation, metadataFileName)
+
+	file, err := os.Open(metadataFile)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("file not found: %v", err)
+		} else {
+			return nil, fmt.Errorf("Error opening file: %v", err)
+		}
+	}
+	defer file.Close()
+
+	var providers dbProviders
+	fileBytes, err := io.ReadAll(file)
+	if err != nil {
+		return nil, fmt.Errorf("error reading file: %v", err)
+	}
+	err = json.Unmarshal(fileBytes, &providers)
+	if err != nil {
+		return nil, fmt.Errorf("cannot unmarshal providers: %v", err)
+	}
+
+	return &providers, nil
+
+}
+
+func displayDBProvidersTable(providers []dbProviderMetadata, output io.Writer) {
+
+	rows := [][]string{}
+	for _, provider := range providers {
+		rows = append(rows, []string{provider.Name, provider.LastSuccessfulRun})
+	}
+
+	table := tablewriter.NewWriter(output)
+	table.SetHeader([]string{"Name", "Last Successful Run"})
+
+	table.AppendBulk(rows)
+	table.Render()
+
+}
+
+func displayDBProvidersJSON(providers *dbProviders, output io.Writer) error {
+
+	encoder := json.NewEncoder(output)
+	encoder.SetEscapeHTML(false)
+	encoder.SetIndent("", " ")
+	err := encoder.Encode(providers)
+	if err != nil {
+		return fmt.Errorf("cannot display json: %v", err)
+	}
+	return nil
+}