Add event-driven or callback-driven SSLIOHandler component for 3rd party engines

[rlebeau]

Indy's default SSLIOHandler component uses OpenSSL. In the future, there will likely be a new SSLIOHandler component added to use Microsoft's SChannel on Windows to replace OpenSSL. But, there are many 3rd party SSL/TLS engines available, which cannot currently be used with Indy without having to write a custom SSLIOHandler class for each one. Perhaps we can make a new general-purpose SSLIOHandler component that is event/callback-driven for key SSL/TLS operations, and then users can plug in whatever SSL/TLS encryption engine they want.