Support deleting all identities from an environment

### Is your feature request related to a problem? Please describe.

Some customers heavily use the Identities section of the dashboard to manage features for their users. For these customers, having a large amount of undesired identities can add noise and they might prefer to start over by deleting all identities from their environment. Some cases that can result in having many undesired identities:

* Application bug, e.g. accidentally calling `identify` with a randomly-generated ID in a loop during development
* Application needs to migrate to a different ID format, and identities in the old format need to be cleaned up

### Describe the solution you'd like.

Add an option in the Environment settings to delete all identities, next to the Delete Environment option. This should require administrator permissions for the environment.

### Describe alternatives you've considered

Deleting identities one by one: annoying to implement and can take a massive amount of API requests depending on how many identities there are.

Cloning an existing environment and starting over with no identities: this requires applications to get new client-side and server-side SDK keys, and breaks existing URLs to the Flagsmith dashboard.

### Additional context

_No response_