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/tests/rpc_tests.rs

@@ -167,7 +167,6 @@ fn test_blocks_by_range_chunked_rpc() {
         let rpc_request = Request::BlocksByRange(BlocksByRangeRequest {
             start_slot: 0,
             count: messages_to_send,
-            step: 0,
         });
 
         let spec = E::default_spec();
@@ -307,7 +306,6 @@ fn test_blocks_by_range_over_limit() {
         let rpc_request = Request::BlocksByRange(BlocksByRangeRequest {
             start_slot: 0,
             count: messages_to_send,
-            step: 0,
         });
 
         // BlocksByRange Response
@@ -405,7 +403,6 @@ fn test_blocks_by_range_chunked_rpc_terminates_correctly() {
         let rpc_request = Request::BlocksByRange(BlocksByRangeRequest {
             start_slot: 0,
             count: messages_to_send,
-            step: 0,
         });
 
         // BlocksByRange Response
@@ -537,7 +534,6 @@ fn test_blocks_by_range_single_empty_rpc() {
         let rpc_request = Request::BlocksByRange(BlocksByRangeRequest {
             start_slot: 0,
             count: 10,
-            step: 0,
         });
 
         // BlocksByRange Response