[lib] introduce type for composable DMOperation

Summary:
ENG-8424.

Improve documenting code and addresses https://phab.comm.dev/D13290#inline-76570.

Depends on D13325

Test Plan: Flow

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13326