Deprecate step param in BlocksByRange RPC request (#3275)

## Issue Addressed

Deprecates the step parameter in the blocks by range request

## Proposed Changes

- Modifies the BlocksByRangeRequest type to remove the step parameter and everywhere we took it into account before
- Adds a new type to still handle coding and decoding of requests that use the parameter

## Additional Info
I went with a deprecation over the type itself so that requests received outside `lighthouse_network` don't even need to deal with this parameter. After the deprecation period just removing the Old blocks by range request should be straightforward

beacon_node/lighthouse_network/src/rpc/methods.rs

@@ -201,6 +201,16 @@ pub struct BlocksByRangeRequest {
 
     /// The number of blocks from the start slot.
     pub count: u64,
+}
+
+/// Request a number of beacon block roots from a peer.
+#[derive(Encode, Decode, Clone, Debug, PartialEq)]
+pub struct OldBlocksByRangeRequest {
+    /// The starting slot to request blocks.
+    pub start_slot: u64,
+
+    /// The number of blocks from the start slot.
+    pub count: u64,
 
     /// The step increment to receive blocks.
     ///
@@ -410,6 +420,12 @@ impl std::fmt::Display for GoodbyeReason {
 }
 
 impl std::fmt::Display for BlocksByRangeRequest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "Start Slot: {}, Count: {}", self.start_slot, self.count)
+    }
+}
+
+impl std::fmt::Display for OldBlocksByRangeRequest {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(
             f,