Use metadata to process data.json requests

By keeping realistic metadata in the dummy, we can process
_offset_-based requests and requests for several instances at the same
time.

Also implement enum requests and add more tests.

schunk_egu_egk_gripper_dummy/config/README.md

@@ -9,6 +9,12 @@ For instance, reading available error codes for the enum `HEX=0x0118` (`DEC=280`
 http://<gripper-ip>/adi/enum.json?inst=280
 ```
 
+## Metadata
+
+```bash
+http://<gripper-ip>/adi/metadata.json?offset=0&count=300
+```
+
 ## Data
 Use this script that reads the data directly from the gripper:
 ```bash