Add lcli indexed-attestations (#3221)

## Proposed Changes

It's reasonably often that we want to manually convert an attestation to indexed form. This PR adds an `lcli` command for doing this, using an SSZ state and a list of JSON attestations (as extracted from a JSON block) as input.

lcli/src/indexed_attestations.rs

@@ -0,0 +1,48 @@
+use clap::ArgMatches;
+use clap_utils::parse_required;
+use state_processing::common::get_indexed_attestation;
+use std::fs::File;
+use std::io::Read;
+use std::path::{Path, PathBuf};
+use types::*;
+
+fn read_file_bytes(filename: &Path) -> Result<Vec<u8>, String> {
+    let mut bytes = vec![];
+    let mut file = File::open(filename)
+        .map_err(|e| format!("Unable to open {}: {}", filename.display(), e))?;
+    file.read_to_end(&mut bytes)
+        .map_err(|e| format!("Unable to read {}: {}", filename.display(), e))?;
+    Ok(bytes)
+}
+
+pub fn run<T: EthSpec>(matches: &ArgMatches) -> Result<(), String> {
+    let spec = &T::default_spec();
+
+    let state_file: PathBuf = parse_required(matches, "state")?;
+    let attestations_file: PathBuf = parse_required(matches, "attestations")?;
+
+    let mut state = BeaconState::<T>::from_ssz_bytes(&read_file_bytes(&state_file)?, spec)
+        .map_err(|e| format!("Invalid state: {:?}", e))?;
+    state
+        .build_all_committee_caches(spec)
+        .map_err(|e| format!("{:?}", e))?;
+
+    let attestations: Vec<Attestation<T>> =
+        serde_json::from_slice(&read_file_bytes(&attestations_file)?)
+            .map_err(|e| format!("Invalid attestation list: {:?}", e))?;
+
+    let indexed_attestations = attestations
+        .into_iter()
+        .map(|att| {
+            let committee = state.get_beacon_committee(att.data.slot, att.data.index)?;
+            get_indexed_attestation(committee.committee, &att)
+        })
+        .collect::<Result<Vec<_>, _>>()
+        .map_err(|e| format!("Error constructing indexed attestation: {:?}", e))?;
+
+    let string_output = serde_json::to_string_pretty(&indexed_attestations)
+        .map_err(|e| format!("Unable to convert to JSON: {:?}", e))?;
+    println!("{}", string_output);
+
+    Ok(())
+}