[lib] Update SendMultimediaMessageRequest type

Summary:
Update SendMultimediaMessageRequest to handle "new" payload option which takes list of MediaMessageServerDBContent instead of a list of strings.

Depends on D5060

Test Plan: NA, no functional changes but addresses a flow issue in subsequent diff

Reviewers: abosh, tomek, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5061