Null-terminated query parameters cause server errors in the Core SDK endpoints

Example Sentry issue: [FLAGSMITH-API-3TZ](https://flagsmith.sentry.io/issues/4585843809/?referrer=github_integration)

```
ValueError: A string literal cannot contain NUL (0x00) characters.
(15 additional frame(s) were not displayed)
...
File "environments/identities/views.py", line 185, in get
.get_or_create(identifier=identifier, environment=request.environment)
```

This should be a problem for every view that accesses query parameters directly.

A quick search yields 8 occurences of this: https://github.com/search?q=repo%3AFlagsmith%2Fflagsmith+query_params.get&type=code

For each of those we need to assess the performance impact of using a serializer (DRF's `CharField` handles null chars).