Added Merkle Proof Generation for Beacon State (#3674)

## Issue Addressed

This PR addresses partially #3651

## Proposed Changes

This PR adds the following methods:

* a new method to trait `TreeHash`, `hash_tree_leaves` which returns all the Merkle leaves of the ssz object.
* a new method to `BeaconState`: `compute_merkle_proof` which generates a specific merkle proof for given depth and index by using the `hash_tree_leaves` as leaves function.

## Additional Info

Now here is some rationale on why I decided to go down this route: adding a new function to commonly used trait is a pain but was necessary to make sure we have all merkle leaves for every object, that is why I just added  `hash_tree_leaves`  in the trait and not  `compute_merkle_proof` as well. although it would make sense it gives us code duplication/harder review time and we just need it from one specific object in one specific usecase so not worth the effort YET. In my humble opinion.

Co-authored-by: Michael Sproul <micsproul@gmail.com>

testing/ef_tests/src/handler.rs

@@ -617,6 +617,30 @@ impl<E: EthSpec + TypeName> Handler for GenesisInitializationHandler<E> {
     }
 }
 
+#[derive(Derivative)]
+#[derivative(Default(bound = ""))]
+pub struct MerkleProofValidityHandler<E>(PhantomData<E>);
+
+impl<E: EthSpec + TypeName> Handler for MerkleProofValidityHandler<E> {
+    type Case = cases::MerkleProofValidity<E>;
+
+    fn config_name() -> &'static str {
+        E::name()
+    }
+
+    fn runner_name() -> &'static str {
+        "light_client"
+    }
+
+    fn handler_name(&self) -> String {
+        "single_merkle_proof".into()
+    }
+
+    fn is_enabled_for_fork(&self, fork_name: ForkName) -> bool {
+        fork_name != ForkName::Base
+    }
+}
+
 #[derive(Derivative)]
 #[derivative(Default(bound = ""))]
 pub struct OperationsHandler<E, O>(PhantomData<(E, O)>);