Fork schedule api (#2525)

## Issue Addressed

Resolves #2524

## Proposed Changes

- Return all known forks in the `/config/fork_schedule`, previously returned only the head of the chain's fork.
- Deleted the `StateId::head` method because it was only previously used in this endpoint.


Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/http_api/src/state_id.rs

@@ -8,10 +8,6 @@ use types::{BeaconState, EthSpec, Fork, Hash256, Slot};
 pub struct StateId(CoreStateId);
 
 impl StateId {
-    pub fn head() -> Self {
-        Self(CoreStateId::Head)
-    }
-
     pub fn slot(slot: Slot) -> Self {
         Self(CoreStateId::Slot(slot))
     }