chore(ai_services): raise exception if api_key None or empty

[Vaibhav159]

1. having api_key check in services where it is required
2. Using self._api_key as a single source of control
3. Custom pipecat exceptions to better error handling

[Vaibhav159]

@aconchillo @markbackman this is up for review, but want to get your thoughts on one thing, i feel setting self._api_key in each service is a bit overkill, since we can put it in core base classes like LLMService/ TTSService and once init is done, can safely refer the same at all places.