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/tests/tests.rs

@@ -465,6 +465,11 @@ fn genesis_validity() {
     // Note: there are no genesis validity tests for mainnet
 }
 
+#[test]
+fn merkle_proof_validity() {
+    MerkleProofValidityHandler::<MainnetEthSpec>::default().run();
+}
+
 #[test]
 fn rewards() {
     for handler in &["basic", "leak", "random"] {