[FirebaseMessaging] multiline/expandable notification support

[felixwortmann]

I know this and this was closed, I do not really understand why though.

I think it would be a very popular requirement to have multiple lines in a notification especially on smaller screens.
Therefore, not adding this feature would require developers do rely on third party libraries which basically makes this one obsolete.

It is understandable to me that you do not have full control over the design and maybe do not add images, but basic styling and more than one line is a really basic feature which should not be hard to implement.