Allow to add/remove/fetch pushers sync

I'll likely add sync methods for add / delete too. Mostly putting this out here to figure out if we want to do this or if we want to leave the sync methods to the app. I'd prefer doing it in the lib as it would be consistent with e.g. what ModemManager does.

added 3 commits

• 097223b3 - client: Clarify return type of cm_client_get_pushers_async
• 04f6ee65 - client: Return empty array when there are no pushers
• 8f5c784b - client: Allow to get/add/remove pushers synchronously

Compare with previous version

changed title from Allow to fetch pushers sync to Allow to add/remove/fetch pushers sync

v2:

• Add methods to add/remove pushers synchronously
• Clarify some docs
• Return empty array when there are no pushers. This makes it a bit nicer to distinguish from the NULL error case

added new-api label

lgtm, I like the style in which the syncs are written :)

Let's merge this then, thanks for the review!

merged