Add an account command to enable/disable validators (#2386)

## Issue Addressed

Resolves #2322 

## Proposed Changes

Adds a `modify` command to `lighthouse account validator` with subcommands to enable and disable specific or all pubkeys.

account_manager/src/validator/modify.rs

@@ -0,0 +1,100 @@
+use account_utils::validator_definitions::ValidatorDefinitions;
+use bls::PublicKey;
+use clap::{App, Arg, ArgMatches};
+use std::{collections::HashSet, path::PathBuf};
+
+pub const CMD: &str = "modify";
+pub const ENABLE: &str = "enable";
+pub const DISABLE: &str = "disable";
+
+pub const PUBKEY_FLAG: &str = "pubkey";
+pub const ALL: &str = "all";
+
+pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
+    App::new(CMD)
+        .about("Modify validator status in validator_definitions.yml.")
+        .subcommand(
+            App::new(ENABLE)
+                .about("Enable validator(s) in validator_definitions.yml.")
+                .arg(
+                    Arg::with_name(PUBKEY_FLAG)
+                        .long(PUBKEY_FLAG)
+                        .value_name("PUBKEY")
+                        .help("Validator pubkey to enable")
+                        .takes_value(true),
+                )
+                .arg(
+                    Arg::with_name(ALL)
+                        .long(ALL)
+                        .help("Enable all validators in the validator directory")
+                        .takes_value(false)
+                        .conflicts_with(PUBKEY_FLAG),
+                ),
+        )
+        .subcommand(
+            App::new(DISABLE)
+                .about("Disable validator(s) in validator_definitions.yml.")
+                .arg(
+                    Arg::with_name(PUBKEY_FLAG)
+                        .long(PUBKEY_FLAG)
+                        .value_name("PUBKEY")
+                        .help("Validator pubkey to disable")
+                        .takes_value(true),
+                )
+                .arg(
+                    Arg::with_name(ALL)
+                        .long(ALL)
+                        .help("Disable all validators in the validator directory")
+                        .takes_value(false)
+                        .conflicts_with(PUBKEY_FLAG),
+                ),
+        )
+}
+
+pub fn cli_run(matches: &ArgMatches, validator_dir: PathBuf) -> Result<(), String> {
+    // `true` implies we are setting `validator_definition.enabled = true` and
+    // vice versa.
+    let (enabled, sub_matches) = match matches.subcommand() {
+        (ENABLE, Some(sub_matches)) => (true, sub_matches),
+        (DISABLE, Some(sub_matches)) => (false, sub_matches),
+        (unknown, _) => {
+            return Err(format!(
+                "{} does not have a {} command. See --help",
+                CMD, unknown
+            ))
+        }
+    };
+    let mut defs = ValidatorDefinitions::open(&validator_dir).map_err(|e| {
+        format!(
+            "No validator definitions found in {:?}: {:?}",
+            validator_dir, e
+        )
+    })?;
+    let pubkeys_to_modify = if sub_matches.is_present(ALL) {
+        defs.as_slice()
+            .iter()
+            .map(|def| def.voting_public_key.clone())
+            .collect::<HashSet<_>>()
+    } else {
+        let public_key: PublicKey = clap_utils::parse_required(sub_matches, PUBKEY_FLAG)?;
+        std::iter::once(public_key).collect::<HashSet<PublicKey>>()
+    };
+
+    // Modify required entries from  validator_definitions.
+    for def in defs.as_mut_slice() {
+        if pubkeys_to_modify.contains(&def.voting_public_key) {
+            def.enabled = enabled;
+            eprintln!(
+                "Validator {} {}",
+                def.voting_public_key,
+                if enabled { "enabled" } else { "disabled" }
+            );
+        }
+    }
+
+    defs.save(&validator_dir)
+        .map_err(|e| format!("Unable to modify validator definitions: {:?}", e))?;
+
+    eprintln!("\nSuccessfully modified validator_definitions.yml");
+    Ok(())
+}