improve validation for azapi_resource data source

[ms-henglu]

Tests:

data "azapi_resource" "rg" {
  type = "Microsoft.Resources/resourceGroups@2024-03-01"
  name = "myRG"
}

data "azapi_resource" "subscription" {
  type = "Microsoft.Resources/subscriptions@2024-03-01"
}

data "azapi_resource" "vnet" {
  type      = "Microsoft.Network/virtualNetworks@2024-03-01"
  name      = "myVnet"
  parent_id = "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/myRG"
}

data "azapi_resource" "vnet2" {
  type        = "Microsoft.Network/virtualNetworks@2024-03-01"
  resource_id = "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/myRG/providers/Microsoft.Network/virtualNetworks/myVnet"
}


# Success! The configuration is valid, but there were some validation warnings as shown above.