Add support for feature flags management API (#1420)

## Description

- Adds the following methods to support the feature flags management API
  - `listFeatureFlags` (`GET /feature-flags`)
  - `getFeatureFlag` (`GET /feature-flags/:slug`)
  - `enableFeatureFlag` (`PUT /feature-flags/:slug/enable`)
  - `disableFeatureFlag` (`PUT /feature-flags/:slug/disable`)
  - `addFlagTarget` (`POST /feature-flags/:slug/targets/:targetId`)
  - `removeFlagTarget` (`DELETE /feature-flags/:slug/targets/:targetId`)
- Updates `FeatureFlag` interface to add `tags`, `enabled` and
`defaultValue` fields
- Updates `FeatureFlag.description` to be a required field to match the
API response

## Documentation

Does this require changes to the WorkOS Docs? E.g. the [API
Reference](https://workos.com/docs/reference) or code snippets need
updates.

```
[x] Yes
```

If yes, link a related docs PR and add a docs maintainer as a reviewer.
Their approval is required.