feat: allow any custom field type in netbox_devices data source

Update the data source `netbox_devices` to use the string based
schema for the `custom_fields` attribute.

This allows for any underlying type to be accessed by using the
`jsondecode()` terraform function.

For example, to access custom fields, the following code can be used.

```terraform
data "netbox_devices" "test" {
	filter {
		name  = "name"
		value = "device1"
	}
}

output "device_field" {
	value = jsondecode(data.netbox_devices.test[0].custom_fields).field1
}
```

docs/data-sources/devices.md

@@ -45,6 +45,7 @@ Read-Only:
 - `comments` (String)
 - `custom_fields` (Map of String)
 - `description` (String)
+- `custom_fields` (String)
 - `device_id` (Number)
 - `device_type_id` (Number)
 - `location_id` (Number)

netbox/data_source_netbox_devices.go

@@ -63,8 +63,10 @@ func dataSourceNetboxDevices() *schema.Resource {
 							Computed: true,
 						},
 						"custom_fields": {
-							Type:     schema.TypeMap,
+							Type:     schema.TypeString,
 							Computed: true,
+							Description: "A JSON string that defines the custom fields as defined under the `custom_fields` key in the object's api." +
+								"The data can be accessed by using the `jsondecode()` function around the `custom_fields` attribute.",
 						},
 						"description": {
 							Type:     schema.TypeString,
@@ -247,9 +249,11 @@ func dataSourceNetboxDevicesRead(d *schema.ResourceData, m interface{}) error {
 		if device.Status != nil {
 			mapping["status"] = *device.Status.Value
 		}
-		if device.CustomFields != nil {
-			mapping["custom_fields"] = device.CustomFields
+		cf, err := handleCustomFieldRead(device.CustomFields)
+		if err != nil {
+			return err
 		}
+		mapping["custom_fields"] = cf
 		if device.Rack != nil {
 			mapping["rack_id"] = device.Rack.ID
 		}

netbox/data_source_netbox_devices_test.go

@@ -190,7 +190,6 @@ func TestAccNetboxDevicesDataSource_CustomFields(t *testing.T) {
 data "netbox_devices" "test" {
 	depends_on = [
 		netbox_device.test,
-		netbox_custom_field.test,
 	]
 
 	filter {
@@ -199,12 +198,18 @@ data "netbox_devices" "test" {
 	}
 }
 
-resource "netbox_custom_field" "test" {
-	name          = "%[1]s"
+resource "netbox_custom_field" "text" {
+	name          = "%[1]s_text"
 	type          = "text"
 	content_types = ["dcim.device"]
 }
 
+resource "netbox_custom_field" "boolean" {
+	name          = "%[1]s_boolean"
+	type          = "boolean"
+	content_types = ["dcim.device"]
+}
+
 resource "netbox_device" "test" {
   name = "%[2]s"
   comments = "thisisacomment"
@@ -219,7 +224,10 @@ resource "netbox_device" "test" {
   location_id = netbox_location.test.id
   status = "staged"
   serial = "ABCDEF"
-  custom_fields = {"${netbox_custom_field.test.name}" = "81"}
+	custom_fields = jsonencode({
+		"${netbox_custom_field.text.name}" = "81"
+		"${netbox_custom_field.boolean.name}" = true
+	})
 }
 `, testField, testName),
 				Check: resource.ComposeTestCheckFunc(
@@ -235,7 +243,7 @@ resource "netbox_device" "test" {
 					resource.TestCheckResourceAttrPair("data.netbox_devices.test", "devices.0.location_id", "netbox_location.test", "id"),
 					resource.TestCheckResourceAttr("data.netbox_devices.test", "devices.0.serial", "ABCDEF"),
 					resource.TestCheckResourceAttr("data.netbox_devices.test", "devices.0.status", "staged"),
-					resource.TestCheckResourceAttr("data.netbox_devices.test", "devices.0.custom_fields."+testField, "81"),
+					resource.TestCheckResourceAttr("data.netbox_devices.test", "devices.0.custom_fields", "{\""+testField+"_boolean\":true,\""+testField+"_text\":\"81\"}"),
 				),
 			},
 		},