MINOR: h2: add h2_set_frame_size() to update the size in a binary frame

MINOR: h2: add h2_set_frame_size() to update the size in a binary frame

This function is called after preparing a frame, in order to update the
frame's size in the frame header. It takes the frame payload length in
argument.

It simply writes a 24-bit frame size into a buffer, making use of the
net_helper functions which try to optimize per platform (this is a
frequently used operation).