Modularize beacon node backend (#4718)

#4669


  Modularize the beacon node backend to make it easier to add new database implementations

beacon_node/src/cli.rs

@@ -1591,5 +1591,14 @@ pub fn cli_app() -> Command {
                 .action(ArgAction::Set)
                 .display_order(0)
         )
+        .arg(
+            Arg::new("beacon-node-backend")
+                .long("beacon-node-backend")
+                .value_name("DATABASE")
+                .value_parser(store::config::DatabaseBackend::VARIANTS.to_vec())
+                .help("Set the database backend to be used by the beacon node.")
+                .action(ArgAction::Set)
+                .display_order(0)
+        )
         .group(ArgGroup::new("enable_http").args(["http", "gui", "staking"]).multiple(true))
 }