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>

beacon_node/beacon_chain/tests/column_verification.rs

@@ -1,5 +1,6 @@
 #![cfg(not(debug_assertions))]
 
+use beacon_chain::custody_context::NodeCustodyType;
 use beacon_chain::test_utils::{
     AttestationStrategy, BeaconChainHarness, BlockStrategy, EphemeralHarnessType,
     generate_data_column_sidecars_from_block, test_spec,
@@ -24,7 +25,7 @@ static KEYPAIRS: LazyLock<Vec<Keypair>> =
 fn get_harness(
     validator_count: usize,
     spec: Arc<ChainSpec>,
-    supernode: bool,
+    node_custody_type: NodeCustodyType,
 ) -> BeaconChainHarness<EphemeralHarnessType<E>> {
     create_test_tracing_subscriber();
     let harness = BeaconChainHarness::builder(MainnetEthSpec)
@@ -34,7 +35,7 @@ fn get_harness(
             ..ChainConfig::default()
         })
         .keypairs(KEYPAIRS[0..validator_count].to_vec())
-        .import_all_data_columns(supernode)
+        .node_custody_type(node_custody_type)
         .fresh_ephemeral_store()
         .mock_execution_layer()
         .build();
@@ -54,8 +55,7 @@ async fn rpc_columns_with_invalid_header_signature() {
         return;
     }
 
-    let supernode = true;
-    let harness = get_harness(VALIDATOR_COUNT, spec, supernode);
+    let harness = get_harness(VALIDATOR_COUNT, spec, NodeCustodyType::Supernode);
 
     let num_blocks = E::slots_per_epoch() as usize;