add configuration for setting blockstore variant

cmd/cocoon/main.go

···
 				EnvVars: []string{"COCOON_DEFAULT_ATPROTO_PROXY"},
 				Value:   "did:web:api.bsky.app#bsky_appview",
 			},
+			&cli.StringFlag{
+				Name:    "blockstore-variant",
+				EnvVars: []string{"COCOON_BLOCKSTORE_VARIANT"},
+				Value:   "sqlite",
+			},
 		},
 		Commands: []*cli.Command{
 			runServe,
···
 	Usage: "Start the cocoon PDS",
 	Flags: []cli.Flag{},
 	Action: func(cmd *cli.Context) error {
+		var bsv server.BlockstoreVariant
+		maybeBsv := cmd.String("blockstore-variant")
+		switch maybeBsv {
+		case "sqlite":
+			bsv = server.BlockstoreVariantSqlite
+		default:
+			panic("invalid blockstore variant!")
+		}
+
 		s, err := server.New(&server.Args{
 			Addr:            cmd.String("addr"),
 			DbName:          cmd.String("db-name"),
···
 			},
 			SessionSecret:       cmd.String("session-secret"),
 			DefaultAtprotoProxy: cmd.String("default-atproto-proxy"),
+			BlockstoreVariant:   bsv,
 		})
 		if err != nil {
 			fmt.Printf("error creating cocoon: %v", err)

server/blockstore_variant.go

···
+package server
+
+type BlockstoreVariant int
+
+const (
+	BlockstoreVariantSqlite = iota
+)

server/server.go

···
 	SessionSecret string
 
 	DefaultAtprotoProxy string
+
+	BlockstoreVariant BlockstoreVariant
 }
 
 type config struct {