Add --semi-supernode support (#8254)

Addresses #8218

A simplified version of #8241 for the initial release.

I've tried to minimise the logic change in this PR, although introducing the `NodeCustodyType` enum still result in quite a bit a of diff, but the actual logic change in `CustodyContext` is quite small.

The main changes are in the `CustdoyContext` struct
* ~~combining `validator_custody_count` and `current_is_supernode` fields into a single `custody_group_count_at_head` field. We persist the cgc of the initial cli values into the `custody_group_count_at_head` field and only allow for increase (same behaviour as before).~~
* I noticed the above approach caused a backward compatibility issue, I've [made a fix](15569bc085) and changed the approach slightly (which was actually what I had originally in mind):
* when initialising, only override the  `validator_custody_count` value if either flag `--supernode` or `--semi-supernode` is used; otherwise leave it as the existing default `0`. Most other logic remains unchanged.

All existing validator custody unit tests are still all passing, and I've added additional tests to cover semi-supernode, and restoring `CustodyContext` from disk.

Note: I've added a `WARN` if the user attempts to switch to a `--semi-supernode` or `--supernode` - this currently has no effect, but once @eserilev column backfill is merged, we should be able to support this quite easily.

Things to test
- [x] cgc in metadata / enr
- [x] cgc in metrics
- [x] subscribed subnets
- [x] getBlobs endpoint


  


Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

testing/ef_tests/src/cases/fork_choice.rs

@@ -16,6 +16,7 @@ use beacon_chain::{
         VerifiedAttestation, obtain_indexed_attestation_and_committees_per_slot,
     },
     blob_verification::GossipVerifiedBlob,
+    custody_context::NodeCustodyType,
     test_utils::{BeaconChainHarness, EphemeralHarnessType},
 };
 use execution_layer::{PayloadStatusV1, json_structures::JsonPayloadStatusV1Status};
@@ -436,7 +437,7 @@ impl<E: EthSpec> Tester<E> {
             .genesis_state_ephemeral_store(case.anchor_state.clone())
             .mock_execution_layer()
             .recalculate_fork_times_with_genesis(0)
-            .import_all_data_columns(true)
+            .node_custody_type(NodeCustodyType::Supernode)
             .mock_execution_layer_all_payloads_valid()
             .build();