[FEATURE] Cannot update custom plugins through the UI

### What happened?

When using the UI to add a newer version of a previously added unofficial plugin, the UI simply responds with "Plugin already exists" and keeps the plugin as it was.

The expected behavior would've been to replace the existing plugin with the new version. In fact, it might be desirable to overwrite the existing plugin with the new one even when the version is the same or lower. (The case of the same version would be particularly useful for developing plugins).

Currently this can be worked around by first uninstalling the plugin in question. This has the downside of resetting its configuration, however.

### How to reproduce?

Create a plugin. Upload it via the web UI. Increase its version. Try to upload it again. Experience error. The plugin remains on the first version.

### Configuration file(s) (yaml or .env)

```YAML
-
```

### Relevant log output

```shell
-
```

### BunkerWeb version

1.6.9

### What integration are you using?

Kubernetes

### Linux distribution (if applicable)

_No response_

### Removed private data

- [x] I have removed all private data from the configuration file and the logs

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct