Implement slashing protection interchange format (#1544)

## Issue Addressed

Implements support for importing and exporting the slashing protection DB interchange format described here:

https://hackmd.io/@sproul/Bk0Y0qdGD

Also closes #1584 

## Proposed Changes

* [x] Support for serializing and deserializing the format
* [x] Support for importing and exporting Lighthouse's database
* [x] CLI commands to invoke import and export
* [x] Export to minimal format (required when a minimal format has been previously imported)
* [x] Tests for export to minimal (utilising mixed importing and attestation signing?)
* [x] Tests for import/export of complete format, and import of minimal format
* [x] ~~Prevent attestations with sources less than our max source (Danny's suggestion). Required for the fake attestation that we put in for the minimal format to block attestations from source 0.~~
* [x] Add the concept of a "low watermark" for compatibility with the minimal format

Bonus!

* [x] A fix to a potentially nasty bug involving validators getting re-registered each time the validator client ran! Thankfully, the ordering of keys meant that the validator IDs used for attestations and blocks remained stable -- otherwise we could have had some slashings on our hands! 😱
* [x] Tests to confirm that this bug is indeed vanquished

validator_client/slashing_protection/src/registration_tests.rs

@@ -0,0 +1,32 @@
+#![cfg(test)]
+
+use crate::test_utils::*;
+use crate::*;
+use tempfile::tempdir;
+
+#[test]
+fn double_register_validators() {
+    let dir = tempdir().unwrap();
+    let slashing_db_file = dir.path().join("slashing_protection.sqlite");
+    let slashing_db = SlashingDatabase::create(&slashing_db_file).unwrap();
+
+    let num_validators = 100u32;
+    let pubkeys = (0..num_validators as usize).map(pubkey).collect::<Vec<_>>();
+
+    let get_validator_ids = || {
+        pubkeys
+            .iter()
+            .map(|pk| slashing_db.get_validator_id(pk).unwrap())
+            .collect::<Vec<_>>()
+    };
+
+    assert_eq!(slashing_db.num_validator_rows().unwrap(), 0);
+
+    slashing_db.register_validators(pubkeys.iter()).unwrap();
+    assert_eq!(slashing_db.num_validator_rows().unwrap(), num_validators);
+    let validator_ids = get_validator_ids();
+
+    slashing_db.register_validators(pubkeys.iter()).unwrap();
+    assert_eq!(slashing_db.num_validator_rows().unwrap(), num_validators);
+    assert_eq!(validator_ids, get_validator_ids());
+}