This PR adds initial version of almost C#-only QUIC implementation as an alternative to MsQuic based implementation. The work originated as my Master Thesis.

Supported QUIC Protocol Features

This implementation is highly in WIP state. The features are at the draft-27 version. The goal was to obtain a minimal working
implementation able to reliably transmit data between client and server. This can be used to evaluate the viability of purely C# implementation for production releases. Most transport-unrelated features are
left unimplemented.

Currently implemented:

• Basic connection establishment
• Encryption
• TLS integration backed by modified OpenSSL
• Sending data, flow control, loss recovery, congestion control
• Stream/Connection termination
• Coalescing packets into a single UDP datagram

Unsupported QUIC Protocol Features

A list of highlights of unimplemented protocol features follows:

• Connection migration
• Stateless reset
• 0-RTT data
• Server Preferred Address
• Version Negotiation
• Address Validation
• Encryption key updates

OpenSSL integration

To function correctly, the implementation requires a custom branch of OpenSSL from Akamai
https://github.com/akamai/openssl/tree/OpenSSL_1_1_1g-quic. The implementation tries to detect that
a QUIC-supporting OpenSSL is present in the path. If not, then a mock implementation is used instead.

The mock implementation can be used for trying the implementation locally, but interoperation with other
QUIC implementations is, of course, possible only with the OpenSSL-based TLS. If you want to make sure your
implementation runs with OpenSSL, define the DOTNETQUIC_OPENSSL environment variable. This will
cause the implementation to throw if proper OpenSSL version cannot be loaded.

Tracing and qvis

The implementation can produce traces that can be consumed by https://qvis.edm.uhasselt.be
visualizer. To collect traces, define DOTNETQUIC_TRACE environment variable to something nonempty. The traces will be
saved in the working directory in a file named [timestamp]-[server|client].qvis.

Disabling encryption

Regardless of TLS used, you can circumvent encryption by defining the DOTNETQUIC_NOENCRYPT environment variable to something nonempty.