Introduce MASQUE, part 1: shared code

This CL introduces the shared code for MASQUE as defined by <https://tools.ietf.org/html/draft-schinazi-masque>. Most of the work here is plumbing in order to override the right methods of the QUIC codebase. The meat of the MASQUE protocol work is in masque_compression_engine.cc.

gfe-relnote: n/a, adds unused code
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+# MASQUE
+
+The files in this directory implement MASQUE as described in
+<https://tools.ietf.org/html/draft-schinazi-masque>.