Split transport and crypto parts into separate files.

Split transport and crypto parts into separate files.

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).