add server::$raw_prepend_ssl

[foxxx0]

With #1422 still open and no viable ideas presented for dealing with the singularly allowed occurance of reuseport for any ip:port quic combination, there is a valid reason to inject raw directives only into the ssl server{} block.

This applies mostly to scenarios where you want the http (plaintext) server{} block to operate autonomously without a forced https redirect.

Without this separate parameter only for the ssl server {} block, you would have to fully duplicate the http and https server{} resources and then only add the desired ssl-only stuff into the raw_prepend parameter of the https server{} resource, which is incredibly cumbersome and error-prone.
Duplicating such configuration blocks is just bad practice, annoying and should be avoided at all costs.

Thus I consider adding an additional raw_prepend_ssl parameter as an acceptable workaround that might even have further use cases that I'm not even aware of at this time.