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/chain_config.rs

@@ -1,3 +1,4 @@
+use crate::custody_context::NodeCustodyType;
 pub use proto_array::{DisallowedReOrgOffsets, ReOrgThreshold};
 use serde::{Deserialize, Serialize};
 use std::str::FromStr;
@@ -118,6 +119,8 @@ pub struct ChainConfig {
     pub invalid_block_roots: HashSet<Hash256>,
     /// Disable the getBlobs optimisation to fetch blobs from the EL mempool.
     pub disable_get_blobs: bool,
+    /// The node's custody type, determining how many data columns to custody and sample.
+    pub node_custody_type: NodeCustodyType,
 }
 
 impl Default for ChainConfig {
@@ -158,6 +161,7 @@ impl Default for ChainConfig {
             data_column_publishing_delay: None,
             invalid_block_roots: HashSet::new(),
             disable_get_blobs: false,
+            node_custody_type: NodeCustodyType::Fullnode,
         }
     }
 }