feat: availability checks

[jrasm91]

• Move MACHINE_LEARNING_PING_TIMEOUT and MACHINE_LEARNING_AVAILABILITY_BACKOFF_TIME to system config (machineLearning.availabilityChecks)
• Improve UI/UX for add/remove machine learning server urls
• Add new system config section for machine learning availability checks
• Avoid sending machine learning urls in every machine learning repository method
• New items are added at the end of the list, instead of the beginning

Before	After

Related #17185

Tested by spinning up a second machine learning container, and shutting one down, then shutting the other one down, then starting the first one again, and then starting the second one again. Doing searches throughout the process and everything worked as expected.