[FEATURE] Add ability to get the RBAC roles currently assigned to a monitor

[smuthukaruppannp]

Is your feature request related to a problem?
Alerting plugin allows specific RBAC roles to be assigned to monitors thru the rbac_roles parameter. Once a monitor has been created, currently there is no way to get the assigned RBAC roles from the get and search monitor APIs.

What solution would you like?
API should return RBAC role information as part of monitor details

What alternatives have you considered?
No alternative except to search the alerting system indices to verify the actual monitor document in OpenSearch

Do you have any additional context?
Add any other context or screenshots about the feature request here.