This commit adds code that implements an interface between ngtcp2 and
a crypto library like OpenSSL/LibreSSL/QuicTLS.
The code mostly follows the structure of the original ngtcp2 crypto
library and shares significant amount of code with it, in particular
the portable its part and the part related to QuicTLS. That is done to
make it easier to update the code from time to time when there are
significant updates to either QUIC or TLS specifications. However,
there are many differences:
1. The code works not only on QuicTLS or LibreSSL, but also on OpenSSL
which, unlike the former two, does not (and, likely, will not) provide
native BoringSSL-style QUIC integration API;
2. The public interface of the library (API) has been significantly
simplified and reduced without sacrificing any functionality;
3. The code avoids using any "magic" constants as most of them can be
derived in runtime. That makes the code *much* easier to follow;
4. A lot of code was significantly simplified as we have some
middleware code that makes a lot of things much easier to do in a
secure way (like 'isc_buffer_t');
5. The code is better hardened and contains more security-related
checks;
6. The code is FIPS compliant. The original code is not suitable for
use on FIPS-certified systems and would fail to initialise on them.
That provides a foundation to implement QUIC networking code across
the whole range of platforms we support and in a unified way without
depending on OpenSSL-specific (or, rather any crypto-library specific)
support for QUIC. Any versions of the crypto-libraries we support
should be sufficient for that provided that they have TLSv1.3 support.
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