Build issues on C++20 (around constructor syntax of templated structs)

[ddevienne]

We are advancing our internal stack to GCC12 and MSVC2022 on C++20, and this breaks WebSocketPP (perhaps because of warning-as-error, I'm not the prime mover here, just the one taking the time to report it).

The colleague that did the port used jcelerier@3e761b5 from https://github.com/jcelerier/websocketpp/tree/develop. But personally I'd prefer the official project to have that fix and a new release. I also see a C++23 deprecation in the open issues as well, which could be part of said minor maintenance release to keep up with newer (stricter?) compilers. WDYT?