Minify slashing protection interchange data (#2380)

## Issue Addressed

Closes #2354

## Proposed Changes

Add a `minify` method to `slashing_protection::Interchange` that keeps only the maximum-epoch attestation and maximum-slot block for each validator. Specifically, `minify` constructs "synthetic" attestations (with no `signing_root`) containing the maximum source epoch _and_ the maximum target epoch from the input. This is equivalent to the `minify_synth` algorithm that I've formally verified in this repository:

https://github.com/michaelsproul/slashing-proofs

## Additional Info

Includes the JSON loading optimisation from #2347

validator_client/slashing_protection/src/lib.rs

@@ -12,7 +12,8 @@ pub mod test_utils;
 pub use crate::signed_attestation::{InvalidAttestation, SignedAttestation};
 pub use crate::signed_block::{InvalidBlock, SignedBlock};
 pub use crate::slashing_database::{
-    InterchangeImportOutcome, SlashingDatabase, SUPPORTED_INTERCHANGE_FORMAT_VERSION,
+    InterchangeError, InterchangeImportOutcome, SlashingDatabase,
+    SUPPORTED_INTERCHANGE_FORMAT_VERSION,
 };
 use rusqlite::Error as SQLError;
 use std::io::{Error as IOError, ErrorKind};