[Feature Request]: Ability to add an `initContainer` for the MySQL StatefulSet

I am unable to run the MySQL database on the k8s cluster as the `mysql` user wherein I set the `securityContext` to `runAsUser` with ID `999`, as available in the `mysql:8` image. Running as root is not an option either, due to security concerns.

This is the log:

```ruby
2023-10-24 19:36:31+00:00 [Note] [Entrypoint]: Entrypoint script for MySQL Server 8.0.27-1debian10 started.
2023-10-24 19:36:31+00:00 [Note] [Entrypoint]: Initializing database files
mysqld: Can't create/write to file '/var/lib/mysql/is_writable' (OS errno 13 - Permission denied)
2023-10-24T19:36:31.447303Z 0 [System] [MY-013169] [Server] /usr/sbin/mysqld (mysqld 8.0.27) initializing of server in progress as process 22
2023-10-24T19:36:31.448251Z 0 [ERROR] [MY-010460] [Server] --initialize specified but the data directory exists and is not writable. Aborting.
2023-10-24T19:36:31.448255Z 0 [ERROR] [MY-013236] [Server] The designated data directory /var/lib/mysql/ is unusable. You can remove all files that the server added to it.
2023-10-24T19:36:31.448284Z 0 [ERROR] [MY-010119] [Server] Aborting
2023-10-24T19:36:31.448376Z 0 [System] [MY-010910] [Server] /usr/sbin/mysqld: Shutdown complete
```

In order to circumvent this, I need to have an `initContainer` where I can override the file permissions on `/var/lib/mysql` to `chmod` it recursively with `999:999` permissions.

This is a request to add the ability to instantiate a new `initContainer` for the MySQL DB StatefulSet.