Standardise FlagNotFound error

**Problem**
Currently, when a flag key isn't found, the SDK returns a generic FlagsmithAPIError with the message `API returned invalid response`.
This forces consumers (cf OpenFeature [crate](https://github.com/open-feature/rust-sdk-contrib/)) to rely on fragile string matching to detect this case.

**Proposed solution**
Add a FlagNotFound variant to ErrorKind and return it from Flags::get_flag when the key doesn't exist in the HashMap.

**Requirements**

- [ ] Add FlagNotFound error kind and return it from Flags::get_flag
- [ ] Update the [OpenFeature provider](https://github.com/open-feature/rust-sdk-contrib) to use the new error kind instead of string matching