fix: TaskProcessingAPI - unregister provider endpoint (#370)

Found this when writing a small test for **nc_py_api**

without this change `$taskProcessingProvider->id` are always `null` 

```php
$taskProcessingProvider = $this->getExAppTaskProcessingProvider($appId, $name);
if ($taskProcessingProvider !== null) {
	$this->mapper->delete($taskProcessingProvider);
	$this->resetCacheEnabled();
	return $taskProcessingProvider;
}
```

Signed-off-by: Alexander Piskun <bigcat88@icloud.com>

lib/Db/TaskProcessing/TaskProcessingProvider.php

@@ -41,6 +41,9 @@ public function __construct(array $params = []) {
 		$this->addType('provider', 'string');
 		$this->addType('custom_task_type', 'string');
 
+		if (isset($params['id'])) {
+			$this->setId($params['id']);
+		}
 		if (isset($params['app_id'])) {
 			$this->setAppId($params['app_id']);
 		}
@@ -63,6 +66,7 @@ public function __construct(array $params = []) {
 
 	public function jsonSerialize(): array {
 		return [
+			'id' => $this->id,
 			'app_id' => $this->appId,
 			'name' => $this->name,
 			'display_name' => $this->displayName,