…by indirect 'addThreadDeviceTask' processing

This commit implements
- POST `addThreadDeviceTask` on api/actions/
- GET api/actions/<actionId>
- DELETE api/actions

The commit also provides integration tests, see tests/restjsonapi.

Please follow the following steps to install/build OTBR.

1. Checkout this PR

2. Build and Install OTBR as usual, e.g. on a Raspberry Pi

3. Restart the OTBR. `sudo systemctl restart otbr-agent`

4. To monitor the log [Errors|Warnings|Info] please open a different terminal instance and use following command:

```
tail -f /var/log/syslog | grep otbr
```

5. Send POST request using BRUNO or CURL, e.g. to join a new device into your network.

```
curl -X POST -H 'Content-Type: application/vnd.api+json' http://localhost:8081/api/actions -d '{"data": [{"type": "addThreadDeviceTask", "attributes": {"eui": "6234567890AACDEA", "pskd": "J01NME", "timeout": 3600}}]}' | jq
```

should return

```
{
  "data": [
    {
      "id": "2d5a8844-b1bc-4f02-93f0-d87b8c3b4e92",
      "type": "addThreadDeviceTask",
      "attributes": {
        "eui": "6234567890AACDEB",
        "pskd": "J01NME",
        "timeout": 3600,
        "status": "pending"
      },
    }
  ]
}
```

6. You may check the status and get the full collection of actions.

```
curl -X GET -H 'Accept: application/vnd.api+json' http://localhost:8081/api/actions | jq
```

should return

```
{
  "data": [
    {
      "id": "2d5a8844-b1bc-4f02-93f0-d87b8c3b4e92",
      "type": "addThreadDeviceTask",
      "attributes": {
        "eui": "6234567890AACDEB",
        "pskd": "J01NME",
        "timeout": 3600,
        "status": "pending"
      }
    }
  ],
  "meta": {
    "collection": {
      "offset": 0,
      "limit": 100,
      "total": 1
    }
  }
}
```

7. View the entry added to the commissioner's table `sudo ot-ctl commissioner joiner table` and expect

```
| ID                    | PSKd                             | Expiration |
+-----------------------+----------------------------------+------------+
|      6234567890aacdea |                           J01NME |    3459027 |
Done
```

8. Start your joiner and after a few seconds repeat above steps 6. and 7.

9. For running the included test script install Bruno-Cli and run the bash script on your border router

```
cd tests/restjsonapi
source ./install_bruno_cli
./test-restjsonapi-server
```

…ation

also includes bug fixes.