fix(fcm): Support arbitrary custom values in the ApnsPayload

[hiranya911]

ApnsPayload can contain any valid JSON values.

Resolves #1023

[lahirumaramba]

Do we need any tests for this? Otherwise, LGTM!

[hiranya911]

Already covered in:

firebase-admin-node/test/unit/messaging/messaging.spec.ts

Lines 3267 to 3304 in 0d72380

	label: 'All APNS properties',
	req: {
	apns: {
	headers: {
	h1: 'v1',
	h2: 'v2',
	},
	payload: {
	aps: {
	alert: {
	title: 'title',
	subtitle: 'subtitle',
	body: 'body',
	titleLocKey: 'title.loc.key',
	titleLocArgs: ['arg1', 'arg2'],
	subtitleLocKey: 'subtitle.loc.key',
	subtitleLocArgs: ['arg1', 'arg2'],
	locKey: 'body.loc.key',
	locArgs: ['arg1', 'arg2'],
	actionLocKey: 'action.loc.key',
	launchImage: 'image',
	},
	badge: 42,
	sound: 'test.sound',
	category: 'test.category',
	contentAvailable: true,
	mutableContent: true,
	threadId: 'thread.id',
	},
	customKey1: 'custom.value',
	customKey2: { nested: 'value' },
	},
	fcmOptions: {
	analyticsLabel: 'test.analytics',
	imageUrl: 'https://example.com/image.png',
	},
	},
	},