Add hook for other apache plugins

[Thalhammer]

I'm currently developing an apache plugin (mod_grpcbackend) that allows a grpc server to provide an HTTP interface without needing to implement a server itself.
I was in the process of adding WebSocket support when I came across this plugin.
The changes I made to this plugin are quite minor but IMHO can be quite helpful for apache plugin writers.
I added a hook which gets called if WebSocketHandler is set to "internal" and there is no dso with this name.
Other plugins can hook into this and check if they are expected to handle it using the provided name.
If so they set the plugin pointer and from there everything is the same as normal.

[jchampio]

@Thalhammer Neat idea. I'm so sorry it's been three years since you opened this PR -- before I start taking a closer look, is this something you'd still find useful?

[jchampio]

(milestone tentative; waiting on author)

[Thalhammer]

Ofc I think its still usefull. In fact I use it everyday in production.

[jchampio]

Great! I want to think about this use case a bit more, and see if I can make your client code even easier with a more invasive change. Definitely a 0.2.x sort of thing.