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/src/cli.rs

@@ -59,6 +59,18 @@ pub fn cli_app() -> Command {
                           helps network resilience by serving all data columns to syncing peers.")
                 .display_order(0)
         )
+        .arg(
+            Arg::new("semi-supernode")
+                .long("semi-supernode")
+                .action(ArgAction::SetTrue)
+                .help_heading(FLAG_HEADER)
+                .conflicts_with("supernode")
+                .help("Run in minimal reconstruction mode. This node will subscribe to and custody \
+                          half of the data columns (enough for reconstruction), enabling efficient \
+                          data availability with lower bandwidth and storage requirements compared to \
+                          a supernode, while still supporting full blob reconstruction.")
+                .display_order(0)
+        )
         .arg(
             Arg::new("malicious-withhold-count")
                 .long("malicious-withhold-count")