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.

lighthouse/Cargo.toml

@@ -16,6 +16,8 @@ modern = ["bls/supranational-force-adx"]
 milagro = ["bls/milagro"]
 # Support minimal spec (used for testing only).
 spec-minimal = []
+# Support Gnosis spec and Gnosis Beacon Chain.
+gnosis = []
 
 [dependencies]
 beacon_node = { "path" = "../beacon_node" }

lighthouse/environment/src/lib.rs

@@ -21,7 +21,7 @@ use std::path::PathBuf;
 use std::sync::Arc;
 use task_executor::{ShutdownReason, TaskExecutor};
 use tokio::runtime::{Builder as RuntimeBuilder, Runtime};
-use types::{EthSpec, MainnetEthSpec, MinimalEthSpec};
+use types::{EthSpec, GnosisEthSpec, MainnetEthSpec, MinimalEthSpec};
 
 #[cfg(target_family = "unix")]
 use {
@@ -87,6 +87,19 @@ impl EnvironmentBuilder<MainnetEthSpec> {
     }
 }
 
+impl EnvironmentBuilder<GnosisEthSpec> {
+    /// Creates a new builder using the `gnosis` eth2 specification.
+    pub fn gnosis() -> Self {
+        Self {
+            runtime: None,
+            log: None,
+            eth_spec_instance: GnosisEthSpec,
+            eth2_config: Eth2Config::gnosis(),
+            eth2_network_config: None,
+        }
+    }
+}
+
 impl<E: EthSpec> EnvironmentBuilder<E> {
     /// Specifies that a multi-threaded tokio runtime should be used. Ideal for production uses.
     ///

lighthouse/src/main.rs

@@ -52,11 +52,12 @@ fn main() {
                 "{}\n\
                  BLS library: {}\n\
                  SHA256 hardware acceleration: {}\n\
-                 Specs: mainnet (true), minimal ({})",
+                 Specs: mainnet (true), minimal ({}), gnosis ({})",
                  VERSION.replace("Lighthouse/", ""),
                  bls_library_name(),
                  have_sha_extensions(),
                  cfg!(feature = "spec-minimal"),
+                 cfg!(feature = "gnosis"),
             ).as_str()
         )
         .arg(
@@ -302,9 +303,11 @@ fn main() {
 
         match eth_spec_id {
             EthSpecId::Mainnet => run(EnvironmentBuilder::mainnet(), &matches, eth2_network_config),
+            #[cfg(feature = "gnosis")]
+            EthSpecId::Gnosis => run(EnvironmentBuilder::gnosis(), &matches, eth2_network_config),
             #[cfg(feature = "spec-minimal")]
             EthSpecId::Minimal => run(EnvironmentBuilder::minimal(), &matches, eth2_network_config),
-            #[cfg(not(feature = "spec-minimal"))]
+            #[cfg(not(all(feature = "spec-minimal", feature = "gnosis")))]
             other => {
                 eprintln!(
                     "Eth spec `{}` is not supported by this build of Lighthouse",