[keyserver/lib] Introduce the thread actions, endpoints, and responders for deleting a custom role

Summary:
Split into separate diffs from D8574, this diff handles introducing the actions, responders, and endpoints for deleting a custom role. This, alongside the core delete function from D8574 will be all
of the back-end needed for deleting roles.

Depends on D8574

Test Plan: This will be tested with the client-side logic introduced in the next diff (split out from D8574)

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8639