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>

lighthouse/tests/validator_client.rs

@@ -194,6 +194,14 @@ fn init_slashing_protections_flag() {
         .with_config(|config| assert!(config.init_slashing_protection));
 }
 
+#[test]
+fn use_long_timeouts_flag() {
+    CommandLineTest::new()
+        .flag("use-long-timeouts", None)
+        .run()
+        .with_config(|config| assert!(config.use_long_timeouts));
+}
+
 // Tests for Graffiti flags.
 #[test]
 fn graffiti_flag() {