Changes to List Voice Agent Groups

added the optional property items/agent_voice_variants to the response with the 200 status

added the optional property items/pre_pickup_prompt_segments to the response with the 200 status

added the new google enum value to the items/pre_pickup_variants/items/voice_provider response property for the response status 200

added the new google enum value to the items/pre_pickup_voice_provider/anyOf[subschema #1: PrePickupVoiceProvider] response property for the response status 200