Currently, an InvalidCertificateHandler can only be specified globally, via SSLManager.
For some applications, it's necessary to use different strategies when handling invalid certificates, depending on the connection.

There is a way in OpenSSL to obtain the related SSL and SSL_CTX
structures from the X509_STORE_CTX* argument passed to SSLManager::verifyCallback().

So a potential solution could be as follows:

1. Poco::Net::Context stores its this pointer in the underlying SSL_CTX object with
   SSL_CTX_set_ex_data().

2. A Poco::Net::Context::Ptr member will be added to Poco::Net::VerificationErrorArgs.

3. In SSLManager::verifyCallback(), obtain the SSL*, from the SSL_CTX*, and
   from that the Context::Ptr. Pass the Context::Ptr to the VerificationErrorArgs.

4. This way, the InvalidCertificateHandler has access to the underlying Context.

5. It’s now also possible to specify a custom InvalidCertificateHandler for each Context.
   SSLManager::verifyCallback() would check whether there is a non-null InvalidCertificateHandler
   specified for the Context. If it is, that InvalidCertificateHandler will be used, otherwise
   the default one from the SSLManager.