rest-api: make request timeout configurable (#4006)

The existing RestApiFetcher implementation used a hard-coded request
timeout of 1s which wouldn't suit all environments. This commit enables
the configuration of request timeout in the RestApiFetcher.

Signed-off-by: Venil Noronha <veniln@vmware.com>

Mirrored from https://github.com/envoyproxy/envoy @ a3364380ae8dbfac692977f3d5a846b7498eb4ba

envoy/api/v2/core/config_source.proto

@@ -44,6 +44,10 @@ message ApiConfigSource {
 
   // For REST APIs, the delay between successive polls.
   google.protobuf.Duration refresh_delay = 3 [(gogoproto.stdduration) = true];
+
+  // For REST APIs, the request timeout. If not set, a default value of 1s will be used.
+  google.protobuf.Duration request_timeout = 5
+      [(validate.rules).duration.gt.seconds = 0, (gogoproto.stdduration) = true];
 }
 
 // Aggregated Discovery Service (ADS) options. This is currently empty, but when