graphiql-spring-boot: allow timeout parameter for SubscriptionClient

[ribizli]

Currently the default timeout paramter for SubscriptionClient is 10s. The default server keep-alive timeout is 15s (graphql.servlet.subscriptions.apollo.keepAliveIntervalSeconds). So the client reopens the connection every 10s because the server keep-alive is late (setting the keep-alive to 9s is a workaround).

Please allow a configuration for the timeout option in the client. E.g. in src/main/resources/graphiql.html:

    var subscriptionsClient = new window.SubscriptionsTransportWs.SubscriptionClient(newUri, { reconnect: true, timeout: ${subscriptionClientTimeout} });