Add API to register card actions

[juliushaertl]

Part of #2630 to allow apps registering custom card actions that will be added to the actions menu in the card sidebar.

The callback will be called with an object following the deck-card rich object string definition from https://github.com/nextcloud/server/blob/078ecac188525490f7bc560c212070167249263b/lib/public/RichObjectStrings/Definitions.php#L247-L283

The action can be registered with:

window.OCA.Deck.registerCardAction({
	label: 'Send mail',
	icon: 'icon-mail',
	callback: (card) => {
		console.debug('Received send mail action for card', card)
	},
})

[juliushaertl]

Apparently we cannot get the absolute URL from the @nextcloud/router package yet for general routes.

[PVince81]

I've raised nextcloud/spreed#5106 for the Talk side

[nickvergessen]

Time to merge