QUIC DSR frontend packetization request sender interface
QUIC DSR frontend packetization request sender interface

Summary:
Just the basic interface to add send instructions to the sender and an
API to invoke it to send it out.

Reviewed By: mjoras

Differential Revision: D26176058

fbshipit-source-id: 3bec3e617f113a19d1e6708ab02f7758263c119c

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