Use JSON by default for Deposit Snapshot Sync (#4397)

Checkpointz now supports deposit snapshot but [they only support returning them in JSON](https://github.com/ethpandaops/checkpointz/issues/74) so I've modified lighthouse to request them in JSON by default.

There's also `get_opt` & `get_opt_with_timeout` methods which seem to expect responses in JSON but were not adding `Accept: application/json` to the request headers so I fixed that as well.

Also the beacon API puts quantities in quotes so I fixed that in the snapshot JSON serialization

consensus/types/src/deposit_tree_snapshot.rs

@@ -30,8 +30,10 @@ impl From<&DepositTreeSnapshot> for FinalizedExecutionBlock {
 pub struct DepositTreeSnapshot {
     pub finalized: Vec<Hash256>,
     pub deposit_root: Hash256,
+    #[serde(with = "serde_utils::quoted_u64")]
     pub deposit_count: u64,
     pub execution_block_hash: Hash256,
+    #[serde(with = "serde_utils::quoted_u64")]
     pub execution_block_height: u64,
 }