DBus API for process monitoring and task managing applications

### Problem

The lack of an API and portals for accessing info of processes and managing them, makes it meaningless to distribute applications with process monitoring and managing functionality as Flatpaks.

### Proposal

As @danirabbit suggested, I'm opening this issue to initiate work on DBus API for process monitoring applications.
Such API should allow to:
* List all processes of host
* Read stats of processes `/proc/%d/stat`
* Manage a selected process (send signals to end, kill, etc. a process)

With a such API it becomes possible to introduce sophisticated portals for a specific, yet still important type of tools. (e.g. [Monitor](https://github.com/stsdc/monitor)).

### Prior Art (Optional)

_No response_