Add the ability to use a private key from an OpenSSL engine.

[fbatogo]

Add a new CMake option of "PAHO_WITH_SSL_ENGINE" that does nothing if "PAHO_WITH_SSL" is not also enabled. Assuming both are enabled, Paho will be built with the ability to use the private key from an OpenSSL engine when authenticating with TLS.

Built and tested against OpenSSL 1.1 and OpenSSL 3.0. OpenSSL 3.0 will, obviously, have compile time warnings about deprecation, but it still works.