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/signed_attestation.rs

@@ -20,6 +20,18 @@ pub enum InvalidAttestation {
     PrevSurroundsNew { prev: SignedAttestation },
     /// The attestation is invalid because its source epoch is greater than its target epoch.
     SourceExceedsTarget,
+    /// The attestation is invalid because its source epoch is less than the lower bound on source
+    /// epochs for this validator.
+    SourceLessThanLowerBound {
+        source_epoch: Epoch,
+        bound_epoch: Epoch,
+    },
+    /// The attestation is invalid because its target epoch is less than or equal to the lower
+    /// bound on target epochs for this validator.
+    TargetLessThanOrEqLowerBound {
+        target_epoch: Epoch,
+        bound_epoch: Epoch,
+    },
 }
 
 impl SignedAttestation {