Fix bug in lcli transition-blocks and improve pretty-ssz (#4513)

## Proposed Changes

- Fix bad `state_root` reuse in `lcli transition-blocks` that resulted in invalid results at skipped slots.
- Modernise `lcli pretty-ssz` to include fork-generic decoders for `SignedBeaconBlock` and `BeaconState` which respect the `--network`/`--testnet-dir` flag.

## Additional Info

Breaking change: the underscore names like `signed_block_merge` are removed in favour of the fork-generic name `SignedBeaconBlock`, and fork-specific names which match the superstruct variants, e.g. `SignedBeaconBlockMerge`.

lcli/src/transition_blocks.rs

@@ -73,9 +73,10 @@ use eth2::{
 };
 use eth2_network_config::Eth2NetworkConfig;
 use ssz::Encode;
+use state_processing::state_advance::complete_state_advance;
 use state_processing::{
-    block_signature_verifier::BlockSignatureVerifier, per_block_processing, per_slot_processing,
-    BlockSignatureStrategy, ConsensusContext, StateProcessingStrategy, VerifyBlockRoot,
+    block_signature_verifier::BlockSignatureVerifier, per_block_processing, BlockSignatureStrategy,
+    ConsensusContext, StateProcessingStrategy, VerifyBlockRoot,
 };
 use std::borrow::Cow;
 use std::fs::File;
@@ -332,10 +333,8 @@ fn do_transition<T: EthSpec>(
 
     // Transition the parent state to the block slot.
     let t = Instant::now();
-    for i in pre_state.slot().as_u64()..block.slot().as_u64() {
-        per_slot_processing(&mut pre_state, Some(state_root), spec)
-            .map_err(|e| format!("Failed to advance slot on iteration {}: {:?}", i, e))?;
-    }
+    complete_state_advance(&mut pre_state, Some(state_root), block.slot(), spec)
+        .map_err(|e| format!("Unable to perform complete advance: {e:?}"))?;
     debug!("Slot processing: {:?}", t.elapsed());
 
     let t = Instant::now();