Currently, retrieving DMs with a given username requires several
actions: first get the current user's ID; list all users and find the ID
of the username; then scan all DM conversations and find the one with
the current user's ID and the username's ID, to finally retrieve the
messages using that conversation ID.
This tool abstracts all that in a single call.
PS: we'll implement a similar tool for multi-person DM conversations in
a subsequent PR.
a00bd4734e