Introduce "upgrade protocol" (websockets)

[etr]

Is your feature request related to a problem? Please describe.
"upgrade protocol" is now supported by libmicrohttpd. We should reach feature parity to allow users to use it (without having to go back to the wrapped library)

Describe why the feature or enhancement you are proposing fits the library.
Reach feature parity on upgrade protocol. This will likely need to be implemented as a new response_type.

Describe the solution you'd like
This will likely need to be implemented as a new response_type.

[neilyoung]

I suppose, there is still no support for websockets?

[LeSpocky]

I suppose, there is still no support for websockets?

libmicrohttpd gained experimental websockets support in December 2021 with version 0.9.74, see their announcment: https://lists.gnu.org/archive/html/libmicrohttpd/2021-12/msg00052.html

[etr]

There is a proposed PR that would add support to websockets here: #236

The only thing blocking it is that it moves the dependency to the experimental version forcefully rather than optionally. I believe this could be fixed easily. I might not have the time right now to fix it, but I'd welcome changes to that PR addressing the issue. That would close a gap.