Support new realtime voices

The [`RealtimeVoice` enum](https://github.com/64bit/async-openai/blob/aeb6d1f82d08b7900c91745fc41327ebc7d2b81b/async-openai/src/types/realtime/session_resource.rs#L79) currently supports just 3 voices.
But, there are currently [8 supported realtime voices](https://platform.openai.com/docs/api-reference/realtime-sessions/session_object#realtime-sessions/session_object-voice).

