api: add OpenAPI spec and example configuration for new node-type

doc/openapi/components/schemas/config/node_obj.yaml

@@ -17,6 +17,7 @@ properties:
 discriminator:
   propertyName: type
   mapping:
+    api: nodes/_api.yaml
     amqp: nodes/_amqp.yaml
     can: nodes/_can.yaml
     comedi: nodes/_comedi.yaml

doc/openapi/components/schemas/config/nodes/_api.yaml

@@ -0,0 +1,3 @@
+allOf:
+- $ref: ../node_obj.yaml
+- $ref: api.yaml

doc/openapi/components/schemas/config/nodes/api.yaml

@@ -0,0 +1,13 @@
+# yaml-language-server: $schema=http://json-schema.org/draft-07/schema
+---
+
+allOf:
+- type: object
+  additionalProperties: true
+  description: |
+    Additional properties can be retrieved via Rest API call:
+
+		```bash
+    curl http://localhost:80/api/v2/universal/api_node_name/config
+    ```
+- $ref: ../node.yaml

etc/examples/nodes/api.conf

@@ -0,0 +1,12 @@
+nodes = {
+	api_node = {
+		type = "api"
+
+		// Additional configuration can be retrieved via Rest API call:
+		//     curl http://localhost:80/api/v2/universal/api_node/config
+		my_setting = "my_value"
+		a = {
+			b = false
+		}
+	}
+}