Adjust beacon node timeouts for validator client HTTP requests (#2352)

## Issue Addressed

Resolves #2313 

## Proposed Changes

Provide `BeaconNodeHttpClient` with a dedicated `Timeouts` struct.
This will allow granular adjustment of the timeout duration for different calls made from the VC to the BN. These can either be a constant value, or as a ratio of the slot duration.

Improve timeout performance by using these adjusted timeout duration's only whenever a fallback endpoint is available.

Add a CLI flag called `use-long-timeouts` to revert to the old behavior.

## Additional Info

Additionally set the default `BeaconNodeHttpClient` timeouts to the be the slot duration of the network, rather than a constant 12 seconds. This will allow it to adjust to different network specifications.


Co-authored-by: Paul Hauner <paul@paulhauner.com>

testing/node_test_rig/src/lib.rs

@@ -4,7 +4,7 @@
 
 use beacon_node::ProductionBeaconNode;
 use environment::RuntimeContext;
-use eth2::{reqwest::ClientBuilder, BeaconNodeHttpClient};
+use eth2::{reqwest::ClientBuilder, BeaconNodeHttpClient, Timeouts};
 use sensitive_url::SensitiveUrl;
 use std::path::PathBuf;
 use std::time::Duration;
@@ -77,6 +77,7 @@ impl<E: EthSpec> LocalBeaconNode<E> {
         Ok(BeaconNodeHttpClient::from_components(
             beacon_node_url,
             beacon_node_http_client,
+            Timeouts::set_all(HTTP_TIMEOUT),
         ))
     }
 }