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/src/data_availability_checker.rs

@@ -868,6 +868,7 @@ impl<E: EthSpec> MaybeAvailableBlock<E> {
 mod test {
     use super::*;
     use crate::CustodyContext;
+    use crate::custody_context::NodeCustodyType;
     use crate::test_utils::{
         EphemeralHarnessType, NumBlobs, generate_rand_block_and_data_columns, get_kzg,
     };
@@ -1201,7 +1202,7 @@ mod test {
         );
         let kzg = get_kzg(&spec);
         let store = Arc::new(HotColdDB::open_ephemeral(<_>::default(), spec.clone()).unwrap());
-        let custody_context = Arc::new(CustodyContext::new(false));
+        let custody_context = Arc::new(CustodyContext::new(NodeCustodyType::Fullnode, &spec));
         let complete_blob_backfill = false;
         DataAvailabilityChecker::new(
             complete_blob_backfill,