Added allowImagePaste prop

Summary: Added allowImagePaste prop on objC side to toggle whether text input supports pasting of images.

Test Plan: Tried pasting an image in every multi-line text input. chat-input-bar was the only one that allowed it, as expected.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D595