Add state-root command and network support to lcli (#4492)

## Proposed Changes

* Add `lcli state-root` command for computing the hash tree root of a `BeaconState`.
* Add a `--network` flag which can be used instead of `--testnet-dir` to set the network, e.g. Mainnet, Goerli, Gnosis.
* Use the new network flag in `transition-blocks`, `skip-slots`, and `block-root`, which previously only supported mainnet.
* **BREAKING CHANGE** Remove the default value of `~/.lighthouse/testnet` from `--testnet-dir`. This may have made sense in previous versions where `lcli` was more testnet focussed, but IMO it is an unnecessary complication and foot-gun today.

lcli/src/transition_blocks.rs

@@ -71,6 +71,7 @@ use eth2::{
     types::{BlockId, StateId},
     BeaconNodeHttpClient, SensitiveUrl, Timeouts,
 };
+use eth2_network_config::Eth2NetworkConfig;
 use ssz::Encode;
 use state_processing::{
     block_signature_verifier::BlockSignatureVerifier, per_block_processing, per_slot_processing,
@@ -94,8 +95,12 @@ struct Config {
     exclude_post_block_thc: bool,
 }
 
-pub fn run<T: EthSpec>(env: Environment<T>, matches: &ArgMatches) -> Result<(), String> {
-    let spec = &T::default_spec();
+pub fn run<T: EthSpec>(
+    env: Environment<T>,
+    network_config: Eth2NetworkConfig,
+    matches: &ArgMatches,
+) -> Result<(), String> {
+    let spec = &network_config.chain_spec::<T>()?;
     let executor = env.core_context().executor;
 
     /*