Similarly to the QUIC API originated in BoringSSL, this API allows
to register custom TLS callbacks for an external QUIC implementation.
See the SSL_set_quic_tls_cbs manual page for details.

Due to a different approach used in OpenSSL 3.5, handling of CRYPTO
frames was streamlined to always write an incoming CRYPTO buffer to
the crypto context.  Using SSL_provide_quic_data(), this results in
transient allocation of chain links and buffers for CRYPTO frames
received in order.  Testing didn't reveal performance degradation of
QUIC handshakes, https://github.com/nginx/nginx/pull/646 provides
specific results.

All definitions now set in ngx_event_quic.h, this includes moving
NGX_QUIC_OPENSSL_COMPAT from autotests to compile time.  Further,
to improve code readability, a new NGX_QUIC_QUICTLS_API macro is
used for QuicTLS that provides old BoringSSL QUIC API.

Signed-off-by: Piotr Sikora <piotr@aviatrix.com>

With this change, the NGX_OPENSSL_NO_CONFIG macro is defined when nginx
is asked to build OpenSSL itself.  And with this macro automatic loading
of OpenSSL configuration (from the build directory) is prevented unless
the OPENSSL_CONF environment variable is explicitly set.

Note that not loading configuration is broken in OpenSSL 1.1.1 and 1.1.1a
(fixed in OpenSSL 1.1.1b, see https://github.com/openssl/openssl/issues/7350).
If nginx is used to compile these OpenSSL versions, configuring nginx with
NGX_OPENSSL_NO_CONFIG explicitly set to 0 might be used as a workaround.

The change allows to compile QUIC with OpenSSL which lacks BoringSSL QUIC API.

This implementation does not support 0-RTT.

OpenSSL library QUIC support cannot be tested at configure time when
using the --with-openssl option so assume it's present if requested.
While here, fixed the error message in case QUIC support is missing.

As was changed in 253cf267f95a.