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>

consensus/types/src/chain_spec.rs

@@ -237,6 +237,19 @@ impl ChainSpec {
         }
     }
 
+    /// Returns a full `Fork` struct for a given `ForkName` or `None` if the fork does not yet have
+    /// an activation epoch.
+    pub fn fork_for_name(&self, fork_name: ForkName) -> Option<Fork> {
+        let previous_fork_name = fork_name.previous_fork().unwrap_or(ForkName::Base);
+        let epoch = self.fork_epoch(fork_name)?;
+
+        Some(Fork {
+            previous_version: self.fork_version_for_name(previous_fork_name),
+            current_version: self.fork_version_for_name(fork_name),
+            epoch,
+        })
+    }
+
     /// Get the domain number, unmodified by the fork.
     ///
     /// Spec v0.12.1