Add possibility to load providers and use them in Poco

[mapogu]

We have need to externally configure and load providers to OpenSSL 3.x and I noticed that current implementation of the Poco::Net::Context does not provide a way to do this. In my understanding the library context needs to be configured for OpenSSL to search for Providers at an appropriate location. In addition to this, the actual external providers need to be loaded and kept alive until shutdown. These two steps have to happen prior to creation of the SSL Context in order to use providers.

Therefore, I've created this patch as a suggestion to add this improvement to Poco.