It contains wrappers for operations with BPF maps and for loading BPF programs.

As with the previous change, it became feasible with feec2cc762f6
that removes ngx_quic_connection_t from ngx_connection_s.

The ngx_http_perl_module module doesn't have a notion of including additional
search paths through --with-cc-opt, which results in compile error incomplete
type 'enum ssl_encryption_level_t' when building nginx without QUIC support.
The enum is visible from quic event headers and eventually pollutes ngx_core.h.

The fix is to limit including headers to compile units that are real consumers.

All code dealing with serializing/deserializing
is moved int srv/event/ngx_event_quic_transport.c/h file.

All macros for dealing with data are internal to source file.

The header file exposes frame types and error codes.

The exported functions are currently packet header parsers and writers
and frames parser/writer.

The ngx_quic_header_t structure is updated with 'log' member. This avoids
passing extra argument to parsing functions that need to report errors.

New files:
    src/event/ngx_event_quic_protection.h
    src/event/ngx_event_quic_protection.c

The protection.h header provides interface to the crypto part of the QUIC:

2 functions to initialize corresponding secrets:

ngx_quic_set_initial_secret()
ngx_quic_set_encryption_secret()

and 2 functions to deal with packet processing:

ngx_quic_encrypt()
ngx_quic_decrypt()

Also, structures representing secrets are defined there.

All functions require SSL connection and a pool, only crypto operations
inside, no access to nginx connections or events.

Currently pool->log is used for the logging (instead of original c->log).

 - events handling moved into src/event/ngx_event_quic.c
 - http invokes once ngx_quic_run() and passes stream callback
 (diff to original http_request.c is now minimal)

 - streams are stored in rbtree using ID as a key
 - when a new stream is registered, appropriate callback is called

 - ngx_quic_stream_t type represents STREAM and stored in c->qs