Implement `SSL_CTX_set_cert_cb` #40

Job	Run time
Clippy nightly (optional)	30s
Clang Tidy	15s
Build+test (stable, ubuntu-latest)	1m 26s
Clippy	54s
Build+test (beta, ubuntu-latest)	1m 26s
Check for documentation errors	36s
Build+test (nightly, ubuntu-latest)	1m 45s
Format	13s
Miri	1m 9s
Valgrind	1m 15s
	9m 29s

Provide feedback