When Adding a plugin to the system, there is no validation check on plugin file

## Bug Report

### Description

When Adding a plugin to the system, there is no validation check on plugin file

### How-to reproduce

Steps to reproduce the behavior:
1. POST 'http://url/v1/plugins' with an empty plugin file

### Expected Behaviour
The system to spit out an validation error saying to provide a plugin file

### Actual Behaviour
The error received was
```json {"uniqueId":"ef0e49ed-e519-4138-bc7e-26cdbba6f9a6","exchangeTime":14,"status":500,"message":"Something went wrong, please check the logs Request-ID: [ef0e49ed-e519-4138-bc7e-26cdbba6f9a6]"} ```

### Logs
```
java.lang.NullPointerException: Cannot invoke "org.springframework.web.multipart.MultipartFile.getBytes()" because the return value of "net.bdavies.babblebot.dto.CreatePluginRequest.getPlugin()" is null

at net.bdavies.babblebot.service.PluginService.lambda$createPlugin$1(PluginService.java:82) ~[babblebot-server-server-3.0.0-rc.22.jar:na]

at net.bdavies.babblebot.api.dto.ResponseBag.from(ResponseBag.java:63) ~[babblebot-server-api-3.0.0-rc.22.jar:na]

at net.bdavies.babblebot.service.PluginService.createPlugin(PluginService.java:67) ~[babblebot-server-server-3.0.0-rc.22.jar:na]

at net.bdavies.babblebot.controller.PluginController.createPlugin(PluginController.java:54) ~[babblebot-server-server-3.0.0-rc.22.jar:na]
```

### Possible Solution
Add validation to the CreatePluginRequest class

### Enviroment

Java Version: 17

BabbleBot-Server Version: 3.0.0-rc.22

Plugins installed: n/a


Related Config Settings: n/a

Start with CreatePluginRequest and the POST handler in PluginController.java, then inspect PluginService.java around lines 67 and 82 where the null plugin reaches getBytes(). Confirm how request validation errors are represented, and verify that an empty plugin request produces a validation response instead of the logged 500 error.

Written by the indexing model from the issue text.