Add validator-manager (#3502)

## Issue Addressed

Addresses #2557

## Proposed Changes

Adds the `lighthouse validator-manager` command, which provides:

- `lighthouse validator-manager create`
    - Creates a `validators.json` file and a `deposits.json` (same format as https://github.com/ethereum/staking-deposit-cli)
- `lighthouse validator-manager import`
    - Imports validators from a `validators.json` file to the VC via the HTTP API.
- `lighthouse validator-manager move`
    - Moves validators from one VC to the other, utilizing only the VC API.

## Additional Info

In 98bcb947c I've reduced some VC `ERRO` and `CRIT` warnings to `WARN` or `DEBG` for the case where a pubkey is missing from the validator store. These were being triggered when we removed a validator but still had it in caches. It seems to me that `UnknownPubkey` will only happen in the case where we've removed a validator, so downgrading the logs is prudent. All the logs are `DEBG` apart from attestations and blocks which are `WARN`. I thought having *some* logging about this condition might help us down the track.

In 856cd7e37d I've made the VC delete the corresponding password file when it's deleting a keystore. This seemed like nice hygiene. Notably, it'll only delete that password file after it scans the validator definitions and finds that no other validator is also using that password file.

validator_client/src/cli.rs

@@ -204,6 +204,26 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     address of this server (e.g., http://localhost:5062).")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("http-allow-keystore-export")
+                .long("http-allow-keystore-export")
+                .help("If present, allow access to the DELETE /lighthouse/keystores HTTP \
+                    API method, which allows exporting keystores and passwords to HTTP API \
+                    consumers who have access to the API token. This method is useful for \
+                    exporting validators, however it should be used with caution since it \
+                    exposes private key data to authorized users.")
+                .required(false)
+                .takes_value(false),
+        )
+        .arg(
+            Arg::with_name("http-store-passwords-in-secrets-dir")
+                .long("http-store-passwords-in-secrets-dir")
+                .help("If present, any validators created via the HTTP will have keystore \
+                    passwords stored in the secrets-dir rather than the validator \
+                    definitions file.")
+                .required(false)
+                .takes_value(false),
+        )
         /* Prometheus metrics HTTP server related arguments */
         .arg(
             Arg::with_name("metrics")