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/lib.rs

@@ -17,6 +17,7 @@ pub mod block_verification_types;
 pub mod builder;
 pub mod canonical_head;
 pub mod chain_config;
+pub mod custody_context;
 pub mod data_availability_checker;
 pub mod data_column_verification;
 mod early_attester_cache;
@@ -55,7 +56,6 @@ pub mod summaries_dag;
 pub mod sync_committee_rewards;
 pub mod sync_committee_verification;
 pub mod test_utils;
-pub mod validator_custody;
 pub mod validator_monitor;
 pub mod validator_pubkey_cache;
 
@@ -84,6 +84,7 @@ pub use block_verification::{
 pub use block_verification_types::AvailabilityPendingExecutedBlock;
 pub use block_verification_types::ExecutedBlock;
 pub use canonical_head::{CachedHead, CanonicalHead, CanonicalHeadRwLock};
+pub use custody_context::CustodyContext;
 pub use events::ServerSentEventHandler;
 pub use execution_layer::EngineState;
 pub use execution_payload::NotifyExecutionLayer;
@@ -99,4 +100,3 @@ pub use state_processing::per_block_processing::errors::{
 };
 pub use store;
 pub use types;
-pub use validator_custody::CustodyContext;