Adds TLSOptions to the OpenSSLContext, which invokes a new private 'configure' method which allows various OpenSSL options to be set. Also add standard verification callbacks and external (via a std::function field in TLSOptions) to allow the user to specify their own method which will perform client certificate checking when a new TLS connection is accepted. Only set up the internal verifyCertCallback if the user-supplied hook is set. All callback hooks are set up in the 'configure' method, and only then if TLSOptions.verifyMode is present (i.e. not defaulted to boost::none), to preserve compatibility for users of this class (e.g. Swift) which want to use OpenSSL's own internal validation functions rather than setting the callbacks. Test-information: Used new code under development in M-Link when setting up a TLSContext, setting verify-mode=require, and set up verifyCertCallback with a local method. Making a client TLS connection which includes a client certificate results in the local verify callback being invoked. Change-Id: Idbb7279e1711fca8123f430bfca0dcfb65bc8da6