api/getMessages: Mark some fields in response as optional.
api/getMessages: Mark some fields in response as optional.

Specify 'found_anchor', 'found_newest', 'found_oldest' as maybe
type. They exist in Zulip back-ends version 1.8 or newer but not
before that. Also document that.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile