RFC: QUIC transport

From the [overview](https://docs.google.com/document/d/1gY9-YNDNAB1eip-RTPbqphgySwSNSDHLq9D5Bty4FSU/edit)
>**What is QUIC?**
>QUIC (Quick UDP Internet Connections) is a new transport protocol for the internet, developed by Google.
>
>QUIC solves a number of transport-layer and application-layer problems experienced by modern web applications, while requiring little or no change from application writers. QUIC is very similar to TCP+TLS+HTTP2, but implemented on top of UDP. Having QUIC as a self-contained protocol allows innovations which aren’t possible with existing protocols as they are hampered by legacy clients and middleboxes.
>
>Key advantages of QUIC over TCP+TLS+HTTP2 include:
>* Connection establishment latency
>* Improved congestion control
>* Multiplexing without head-of-line blocking
>* Forward error correction
>* Connection migration

[IETF drafts](https://datatracker.ietf.org/wg/quic/documents/)

[50-minute overview video](https://www.youtube.com/watch?v=hQZ-0mXFmk8)

In the matrix client-server spec, [it mentions this](https://matrix.org/docs/spec/client_server/r0.2.0.html#api-standards):

>More efficient optional transports will in future be supported as optional extensions - e.g. a packed binary encoding over stream-cipher encrypted TCP socket for low-bandwidth/low-roundtrip mobile usage.

I propose QUIC be selected and implemented as such an extension. It seems to be perfectly suited for Matrix: interactive, low-latency, low-bandwidth, multiplexed, encrypted, with natural support for roaming connections (e.g. smartphones). It's a fairly straightforward protocol to implement (see an [example Go implementation](https://github.com/lucas-clemente/quic-go)), and appears to support user-defined congestion control algorithms.

Further, there is a [rather straightforward mapping of HTTP2 onto QUIC](https://datatracker.ietf.org/doc/draft-ietf-quic-http/).

Start with the Matrix client-server specification's API standards section, then review the linked IETF QUIC drafts and the example Go implementation. The issue proposes selecting and implementing QUIC as an optional Matrix transport extension; done would require a defined protocol mapping and corresponding specification changes, but the scope is not specified here.

Written by the indexing model from the issue text.