Add 'multitenant' mode for elasticsearch and postgresql db resources

[luisarojas]

Add support for multitenant flag to be used as host_flavor.id property in the ibm_database resource.

For example:

resource "ibm_database" "elasticsearch_enterprise_multitenant_flavor_auto_cpu_scale" {
  name     = "elasticsearch-enterprise-multitenant-flavor-auto-cpu-scale"
  service  = "databases-for-elasticsearch"
  plan     = "enterprise"
  location = "us-south"
  group {
    group_id = "member"
    host_flavor {
      id = "multitenant"
    }
    disk { # >= 5120 and <= 4194304 in increments of 1024
      allocation_mb = 5120
    }
    memory { # >= 4096 and <= 114688 in increments of 128
      allocation_mb = 4096
    }
    cpu {                  # >= 0 and <= 28 in increments of 1
      allocation_count = 0 # Automatically allocate based on a 1:8 ration with RAM
    }
  }
}

Would have a cost breakdown:

 ibm_database.elasticsearch_enterprise_multitenant_flavor_auto_cpu_scale                                    
 ├─ RAM (3 members)                                                                12  GB           $194.05 
 ├─ Disk (3 members)                                                               15  GB             $9.42 
 └─ Virtual Processor Cores (3 members)                                           1.5  CPU           $48.51 

Expected cost, provided by user (IBM DA team)