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.

lcli/src/main.rs

@@ -38,7 +38,7 @@ fn main() {
                 .value_name("STRING")
                 .takes_value(true)
                 .required(true)
-                .possible_values(&["minimal", "mainnet"])
+                .possible_values(&["minimal", "mainnet", "gnosis"])
                 .default_value("mainnet")
                 .global(true),
         )
@@ -665,6 +665,7 @@ fn main() {
         .and_then(|eth_spec_id| match eth_spec_id {
             EthSpecId::Minimal => run(EnvironmentBuilder::minimal(), &matches),
             EthSpecId::Mainnet => run(EnvironmentBuilder::mainnet(), &matches),
+            EthSpecId::Gnosis => run(EnvironmentBuilder::gnosis(), &matches),
         });
 
     match result {