Native support for Gnosis Beacon Chain network (#2931)

## Proposed Changes

Add a new hardcoded spec for the Gnosis Beacon Chain.
Ideally, official Lighthouse executables will be able to connect to the gnosis beacon chain from now on, using `--network gnosis` CLI option.

boot_node/src/lib.rs

@@ -63,6 +63,9 @@ pub fn run(
         EthSpecId::Mainnet => {
             main::<types::MainnetEthSpec>(lh_matches, bn_matches, eth2_network_config, log)
         }
+        EthSpecId::Gnosis => {
+            main::<types::GnosisEthSpec>(lh_matches, bn_matches, eth2_network_config, log)
+        }
     } {
         slog::crit!(slog_scope::logger(), "{}", e);
     }