[keyserver] Introduce assignMessageContainerToMedia(...) in upload-updaters

Summary:
Pretty much assignMedia(...) but accepts mediaMessageContents: $ReadOnlyArray<MediaMessageServerDBContent> instead of mediaIDs: $ReadOnlyArray<string>.

The intention is to use assignMessageContainerToMedia(...) in the "new"/non-legacy multimediaMessageCreationResponder(...)

Depends on D5086

Test Plan:
Later in the stack this function is "consumed" from multimediaMessageCreationResponder(...). I set some breakpoints and confirmed that values were as expected. I also checked the uploads table in my local MySQL DB and found that the container column was assigned as expected:

Reviewers: abosh, tomek, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5087