Add JSON log format to logging configuration (#2982)

Now patroni can be configured as bellow to log in json format.

```yaml
log:
  type: json
  format:
    - asctime: '@timestamp'
    - levelname: level
    - message
    - module
    - name: logger_name
  static_fields:
    app: patroni
```

This config produce this log:

```json
{
  "@timestamp": "2023-12-14 19:51:24,872",
  "level": "INFO",
  "message": "Lock owner: None; I am postgresql1",
  "module": "ha",
  "app": "patroni",
  "logger_name": "patroni.ha"
}
```

tests/test_config.py

@@ -35,6 +35,7 @@ class TestConfig(unittest.TestCase):
             'PATRONI_NAMESPACE': '/patroni/',
             'PATRONI_SCOPE': 'batman2',
             'PATRONI_LOGLEVEL': 'ERROR',
+            'PATRONI_LOG_FORMAT': '["message", {"levelname": "level"}]',
             'PATRONI_LOG_LOGGERS': 'patroni.postmaster: WARNING, urllib3: DEBUG',
             'PATRONI_LOG_FILE_NUM': '5',
             'PATRONI_CITUS_DATABASE': 'citus',