interfaces: mark optional response fields

[BruceMacD]

These fields will only be present on some responses, such as the final response in the stream or a remote model response.

resolves #262

[jmorganca]

Do you know what the cause is for these CI warnings?

[hopperelec]

Could it be better to be more explicit and have two interfaces PartialChatResponse and DoneChatResponse? That way, the developer can use done for type-checking of all the other properties, rather than having to check individually for each one.