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>

beacon_node/http_api/tests/tests.rs

@@ -9,7 +9,7 @@ use discv5::enr::{CombinedKey, EnrBuilder};
 use environment::null_logger;
 use eth2::Error;
 use eth2::StatusCode;
-use eth2::{types::*, BeaconNodeHttpClient};
+use eth2::{types::*, BeaconNodeHttpClient, Timeouts};
 use eth2_libp2p::{
     rpc::methods::MetaData,
     types::{EnrBitfield, SyncState},
@@ -35,6 +35,7 @@ use types::{
 
 type E = MainnetEthSpec;
 
+const SECONDS_PER_SLOT: u64 = 12;
 const SLOTS_PER_EPOCH: u64 = 32;
 const VALIDATOR_COUNT: usize = SLOTS_PER_EPOCH as usize;
 const CHAIN_LENGTH: u64 = SLOTS_PER_EPOCH * 5 - 1; // Make `next_block` an epoch transition
@@ -213,6 +214,7 @@ impl ApiTester {
                 listening_socket.port()
             ))
             .unwrap(),
+            Timeouts::set_all(Duration::from_secs(SECONDS_PER_SLOT)),
         );
 
         Self {
@@ -327,6 +329,7 @@ impl ApiTester {
                 listening_socket.port()
             ))
             .unwrap(),
+            Timeouts::set_all(Duration::from_secs(SECONDS_PER_SLOT)),
         );
 
         Self {