add ssl engine

[shawshank119]

add ssl engine mode for load private key.

[fpagliughi]

Hey, thanks for the PR, and sorry for the slow response.

Obviously this is blocked by the PR to the Paho C library:
eclipse-paho/paho.mqtt.c#1247

Also, the changes to the ssl_options constructor parameter order would technically be a breaking change. It would be better to add the parameters with default values at the end of the parameter list. That wouldn't break existing code. But, obviously, the C update needs to be addressed first.

[Pascal-Fremaux]

@fpagliughi Be careful that the ENGINE lib support is deprecated in Open SSL 3.0 and removed from further 3.x releases.
So this should be limited to OpenSSL 1.x usage.
This is replaced by the Provider plugins with new APIs.
https://docs.openssl.org/3.0/man7/migration_guide/#upgrading-from-openssl-102

[fpagliughi]

@Pascal-Fremaux , thanks for the heads up. I never got around to looking at this, waiting on the C lib, but I wouldn’t look to implement this if it’s just a 1.x thing. OpenSSL is one library that I think people shouldn’t keep using old versions. Gotta stay ahead of the exploits.