feature: Configure URLRequest timeout interval (apollographql/apollo-ios-dev#618)

Author: calvincestari

Sources/Apollo/HTTPRequest.swift

@@ -85,8 +85,14 @@ open class HTTPRequest<Operation: GraphQLOperation>: Hashable {
   /// - Throws: Any error in creating the request
   /// - Returns: The URL request, ready to send to your server.
   open func toURLRequest() throws -> URLRequest {
-    var request = URLRequest(url: self.graphQLEndpoint)
-    
+    var request: URLRequest
+
+    if let configContext = context as? any RequestContextTimeoutConfigurable {
+      request = URLRequest(url: self.graphQLEndpoint, timeoutInterval: configContext.requestTimeout)
+    } else {
+      request = URLRequest(url: self.graphQLEndpoint)
+    }
+
     for (fieldName, value) in additionalHeaders {
       request.addValue(value, forHTTPHeaderField: fieldName)
     }

Sources/Apollo/RequestContext.swift

@@ -10,3 +10,15 @@ import ApolloAPI
 /// This allows the various interceptors to make modifications, or perform actions, with information
 /// that they cannot get just from the existing operation. It can be anything that conforms to this protocol.
 public protocol RequestContext {}
+
+/// A request context specialization protocol that specifies options for configuring the timeout of a `URLRequest`.
+///
+/// A `RequestContext` object can conform to this protocol to provide a custom `requestTimeout` for an individual request.
+/// If the `RequestContext` for a request does not conform to this protocol, the default request timeout of `URLRequest` will be used.
+public protocol RequestContextTimeoutConfigurable: RequestContext {
+  /// The timeout interval specifies the limit on the idle interval allotted to a request in the process of
+  /// loading. This timeout interval is measured in seconds.
+  ///
+  /// The value of this property will be set as the `timeoutInterval` on the `URLRequest` created for this GraphQL request.
+  var requestTimeout: TimeInterval { get }
+}