Add custom GPT methods

[adamlui]

Discussed in #252

^{Originally posted by pranav-bhatt January 13, 2024:}

As the chat GPT page has a ton of new features pertaining to GPT versions and custom GPTs, it would be very handy to have the ability to get or set chat context (amongst multiple historical chats or to a particular custom GPT) from the library.

{ Thank you so much for the time and effort you've put into making this library possible :) }