based on discussion in #156885 this issue proposes extension defined additional actions for Comment Threads. (See image below for reference).

Motivation

To improve the efficiency of work in Code Reviews, extensions providing CommentThreads would like to add direct resolution of Comments without the need to open the Reply button area (Done in the image).
Additionally, there may be extra actions that may be attached to comment, such as proposed edit that can be applied, which conceptually belong together with the Comment, but at the moment do not have a discoverable area to be placed in. (At the moment actions connected to the whole CommentThread could be either placed in the title to be directly visible, relying on icon to be discovered, or in the reply area which is concealed by the additional click)

Technical proposal

To allow extensions to define additionalActions which are always visible in the CommentThread, a new area should be defined, right under the commentReply (Reply Button Area at the bottom of the CommentThread).
This area would contain separator & extension defined actions.
Extensions can define both single commands and submenus.

Submenus will be rendered as Dropdown buttons, where the first action in the submenu will be treated as the primary action - due to the submenus not carrying Command.
If submenu contains only 1 action, then a single button will be created instead of dropdown button.
In case of no action attached to submenu, the submenu action would be displayed.

NOTE: if there are doubts about AdditionalActions being the correct name for this new area, we should change it.