Hook for sending APP packets

[wowaser]

Hello, this is my proposition for #155
What do you think, @jrsnen?
I tested it and it works for me

[jrsnen]

Hi.

As a general idea, I think a hook based approach is the best solution for the APP, since I think it best mimics how details are gathered for other RTCP reports. That being said,

1. can you modify this, so you also keep the old function. As a library we try to not remove functionality if possible and maybe there is a scenario where that style of API is the best.

2. would install_send_app_hook be a better name? I started thinking that sending is a more important distinction than app, and therefore it would be nice if it was before app in name. Other name would be install_outgoing_app_hook().

3. I think giving the map as a parameter is somewhat more complicated than it needs to be from the user's perspective. I would do it so that the user could install multiple hooks, each for one packet. Something like: install_send_app_hook(std::function < std::shared_ptr<rtcp_app_packet>()> app_sending), which then would be saved to an array where each function would be called when sending an RTCP report.

4. can you add a function for removing the APP hook/all APP hooks?

Points 1 and 3 need to be done for this to be accepted. I'm open to hearing opinions on point 2. Point 4 would be nice, but can be left to a later date.

BR, Joni

[wowaser]

This is deprecated and resolved by #187