Python: add truncation methods to chat history

[eavanvalkenburg]

In a long chat conversation you want to be able to truncate the message history, either by deleting specific messages, oldest messages or by replacing (a set of) messages with a summary, this should be easy and transparent. Related to #3559

[eavanvalkenburg]

This can now be started on the Chathistory object, @matthewbolmatt this would be a great one for the community to pick up!

[alliscode]

This is a dupe of #6770