[lib] Introduce MessageSpec.mergeIntoPrecedingRobotextMessageItem

Summary:
This function will allow us to merge RobotextMessageInfos into preceding (older) RobotextChatMessageInfoItems.

The merging is implemented on the spec of the newer RobotextChatMessageInfoItem.

Depends on D13686

Test Plan: Tested in combination with the following diffs, where I construct RobotextChatMessageInfoItem with an array of messageInfos to represent combined messages (ENG-9559

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13687