Add FrameBuilder for building frames with target packet length

[sippejw]

Adds FrameBuilder that can adjust PADDING frame sizes to achieve a target length. Completes issue #38

[sippejw]

Marked as draft because this doesn't do exactly what I want yet. We want to be able to specify the length of the entire packet (payload and header) and then add padding accordingly. The FrameBuilder does not have access to the header length (I don't think the header has even been built at this point) so I don't yet see a clean way to do this.

[gaukas]

I see what you meant there. Do we have any captured samples showing the variable length of Initial Packet header impacting the length of the frame section?

Actually we DO have the complete header ready at the point we build frame sections (see (*uPacketPacker).appendInitialPacket and (*uPacketPacker).MarshalInitialPacketPayload)

uquic/u_packet_packer.go

Line 168 in cb2c7f1

func (p *uPacketPacker) appendInitialPacket(buffer *packetBuffer, header *wire.ExtendedHeader, pl payload, encLevel protocol.EncryptionLevel, sealer sealer, v protocol.Version) (*longHeaderPacket, error) {

uquic/u_packet_packer.go

Line 223 in cb2c7f1

func (p *uPacketPacker) MarshalInitialPacketPayload(pl payload, v protocol.Version) ([]byte, error) {

---

I think originally quic-go does calculate the padding length at run-time. See

uquic/packet_packer.go

Line 769 in cb2c7f1

func (p *packetPacker) appendLongHeaderPacket(buffer *packetBuffer, header *wire.ExtendedHeader, pl payload, padding protocol.ByteCount, encLevel protocol.EncryptionLevel, sealer sealer, v protocol.Version) (*longHeaderPacket, error) {

from which I derived (*uPacketPacker).appendInitialPacket.

[gaukas]

Looking good.

Can we move the definition and implementation of QUICPacket to after the end of `func (qfs QUICFrames) BuildFromFrames(frames []byte) (payload []byte, err error)? Just to keep each type definition and implementation together in one place.

[gaukas]

Suggested change

	func (qp QUICPacket) Build(cryptoData []byte) (payload []byte, err error) {
	func (qp *QUICPacket) Build(cryptoData []byte) (payload []byte, err error) {

Let's make *QUICPacket instead of QUICPacket the one interfacing QUICFrameBuilder, just to align with the style of *QUICRandomFrames. (QUICFrames is an exception as it is a slice instead of a ground-truth struct)