Enable Inventory and Custom threshold rules in serverless

[paulb-elastic]

Infra UI in serverless needs to include the Inventory threshold rule and the Custom threshold rule needs to be made available under the Observability wide Rules:

Implementation

• Enable these feature flags
  • xpack.infra.featureFlags.inventoryThresholdAlertRuleEnabled
  • xpack.infra.featureFlags.alertsAndRulesDropdownEnabled
  • xpack.observability.unsafe.thresholdRule.enabled

Testing

• Deploy the PR in oblt serverless
• End-to-end test the rules work
  • Including RBAC, not super user

User acceptance

• Share environment with @roshan-elastic and @vinaychandrasekhar to perform user acceptance testing and sign off

[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)

[paulb-elastic]

Updated the title and main description a little to clarity that Inventory threshold will be added to the Infra UI in serverless, and Custom threshold will be made available under the Observability rules