Allow to configure Cassandra's request throttling

[snicoll]

Follow-up of #18621

The maxQueueSize property in Spring Boot 2.2 has been mapped to REQUEST_THROTTLER_MAX_QUEUE_SIZE. There is a case there to improve the support around request throttling by creating a dedicated namespace and offer additional configuration options.

[adutra]

Request throttling in driver 4.0 has indeed a few more configurable options, see here for details.