Add internal serialization of debug_data to HTTP/2 GOAWAY frames

[seanmonstar]

It would be useful to be able to send the additional debug_data in HTTP/2 GOAWAY frames. The frame currently has a field to hold it, and uses that when decoding, but if we try to send a frame with extra data, it gets ignored. Here's some steps to get this done:

• Set the payload length and serialize the debug_data in GoAway::encode
• Add a fn data(self, b: Into<Bytes>) method to the frames test support.
• Add a unit test to h2-tests showing debug data was serialized.

[Herbstein]

I picked this up in the linked PR. Though I do expect a bit of guidance is necessary to bring it up to snuff