[lib] Introduce tMediaMessage[Photo/Video/Media] to validate requests to textMessageCreationResponder

Summary:
We're modifying textMessageCreationResponder to be able to accept a list of MediaMessageServerDBContents as one of the arguments.

Before we build the new validator for textMessageCreationResponder we introduce these basic types that we will build upon in subsequent diffs.

Depends on D4985

Test Plan: Will add tests in next diff

Reviewers: abosh, jacek, tomek, ashoat

Reviewed By: abosh, tomek

Differential Revision: https://phab.comm.dev/D4990