add env auth to drive provider

This change provides the ability to pass `env_auth` as a parameter to
the drive provider. This enables the provider to pull IAM
credentials from the environment or instance metadata. Previously if no
auth method was given it would default to requesting oauth.

backend/drive/drive.go

@@ -202,7 +202,7 @@ func init() {
 					m.Set("root_folder_id", "appDataFolder")
 				}
 
-				if opt.ServiceAccountFile == "" && opt.ServiceAccountCredentials == "" {
+				if opt.ServiceAccountFile == "" && opt.ServiceAccountCredentials == "" && !opt.EnvAuth {
 					return oauthutil.ConfigOut("teamdrive", &oauthutil.Options{
 						OAuth2Config: driveConfig,
 					})
@@ -598,6 +598,18 @@ resource key is no needed.
 			// Encode invalid UTF-8 bytes as json doesn't handle them properly.
 			// Don't encode / as it's a valid name character in drive.
 			Default: encoder.EncodeInvalidUtf8,
+		}, {
+			Name:     "env_auth",
+			Help:     "Get IAM credentials from runtime (environment variables or instance meta data if no env vars).\n\nOnly applies if service_account_file and service_account_credentials is blank.",
+			Default:  false,
+			Advanced: true,
+			Examples: []fs.OptionExample{{
+				Value: "false",
+				Help:  "Enter AWS credentials in the next step.",
+			}, {
+				Value: "true",
+				Help:  "Get GCP IAM credentials from the environment (env vars or IAM).",
+			}},
 		}}...),
 	})
 
@@ -654,6 +666,7 @@ type Options struct {
 	SkipDanglingShortcuts     bool                 `config:"skip_dangling_shortcuts"`
 	ResourceKey               string               `config:"resource_key"`
 	Enc                       encoder.MultiEncoder `config:"encoding"`
+	EnvAuth                   bool                 `config:"env_auth"`
 }
 
 // Fs represents a remote drive server
@@ -1122,6 +1135,12 @@ func createOAuthClient(ctx context.Context, opt *Options, name string, m configm
 		if err != nil {
 			return nil, fmt.Errorf("failed to create oauth client from service account: %w", err)
 		}
+	} else if opt.EnvAuth {
+		scopes := driveScopes(opt.Scope)
+		oAuthClient, err = google.DefaultClient(ctx, scopes...)
+		if err != nil {
+			return nil, fmt.Errorf("failed to create client from environment: %w", err)
+		}
 	} else {
 		oAuthClient, _, err = oauthutil.NewClientWithBaseClient(ctx, name, m, driveConfig, getClient(ctx, opt))
 		if err != nil {