[RAC] whitelist xpack.ruleRegistry.write.disabledRegistrationContexts flag on cloud

[mgiota]

As part of #119217 we need to whitelist xpack.ruleRegistry.write.disabledRegistrationContexts flag so that our cloud users can disable observability rule types in a more granular way.

The registration contexts we use are:

• observability.logs
• observability.metrics
• observability.apm
• observability.uptime

Here's an example of how we whitelisted the rest RAC flags and here's the documentation on how to whitelist settings on cloud.