Add flag to import all attestations (#1941)

## Issue Addressed

NA

## Proposed Changes

Adds the `--import-all-attestations` flag which tells the `network::AttestationService` to import/aggregate all attestations after verification (instead of only ones for subnets that are relevant to local validators).

This is useful for testing/debugging and also for creating back-up nodes that should be all cached up and ready for any validator.

## Additional Info

NA

beacon_node/src/config.rs

@@ -373,6 +373,10 @@ pub fn set_network_config(
         config.subscribe_all_subnets = true;
     }
 
+    if cli_args.is_present("import-all-attestations") {
+        config.import_all_attestations = true;
+    }
+
     if let Some(listen_address_str) = cli_args.value_of("listen-address") {
         let listen_address = listen_address_str
             .parse()