Add PDS host argument to blob export (#884)

This PR introduces the `--pds-host` argument to the `blob export`
command chain.

This was needed in order to export the blobs from an old PDS when by DID
document was pointing to the new one already.

cmd/goat/blob.go

@@ -28,6 +28,10 @@ var cmdBlob = &cli.Command{
 					Aliases: []string{"o"},
 					Usage:   "directory to store blobs in",
 				},
+				&cli.StringFlag{
+					Name:    "pds-host",
+					Usage:   "URL of the PDS to export blobs from (overrides DID doc)",
+				},
 			},
 			Action: runBlobExport,
 		},
@@ -73,9 +77,14 @@ func runBlobExport(cctx *cli.Context) error {
 		return err
 	}
 
+	pdsHost := cctx.String("pds-host")
+	if pdsHost == "" {
+		pdsHost = ident.PDSEndpoint()
+	}
+
 	// create a new API client to connect to the account's PDS
 	xrpcc := xrpc.Client{
-		Host: ident.PDSEndpoint(),
+		Host: pdsHost,
 	}
 	if xrpcc.Host == "" {
 		return fmt.Errorf("no PDS endpoint for identity")