Enforce ownership for flagsmith_feature resources

When creating a flag, there is an owners attribute:
```
owners: List of user IDs representing the owners of the feature.
```
However, it's not mandatory, nor it is simple to retrieve what the ID is (need to enable JSON mode to find the ID).

Is it possible to either:
* Enforce an owner is set
* Default a flag owner automatically (to the user's API token)
* Pass in a user's email, instead of an ID