Add obs-websocket headers-only libraries on Linux and add their finders

[tytan652]

Description

Related to:

• https://github.com/obsproject/obs-deps/pull/155

Dependency of:

• https://github.com/obsproject/obs-websocket/pull/1097

Adds nlohmann JSON, WebSocket++ and Asio as packages for Ubuntu and modules for Flatpak.

Adds finders for Asio and WebSocket++ (the one provided by WebSocket++ has a broken version check) for Linux, Windows and macOS.

Those libraries are not cleaned up in the Flatpak bundle to allow plugin to re-use the exact same versions of these libraries.

Motivation and Context

Reduce the number of submodules in obs-studio repo.

How Has This Been Tested?

obs-studio CI on my fork (https://github.com/tytan652/obs-studio/pull/10) with obs-websocket modified to be aware of obs-deps and custom-made finders for Asio and WebSocket++.

Types of changes

• CI and CMake additions

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.