feat/build: cmake packages

[G4Vi]

This applies a somewhat more modern approach to cmake dependency handling. The next step after this is revamping the README with a Getting Started guide.

• extism and jsoncpp are now always linked via cmake targets instead of sometimes doing a filename search of libraries, whether they are included with add_subdirectory, find_package or FetchContent.
• Removed submodules. Projects wishing to do an in-tree build should include extism, jsoncpp, and cpp-sdk all in the same directory. Including them within cpp-sdk made having a flat dependency tree impossible.
• Added building and making available extism and jsoncpp dependencies with FetchContent. If the build is not an in-tree build and find_package fails, they are downloaded and built.
• Added extism-cpp CMake package to more easily integrate the cpp-sdk with non-intree build projects.
• GTest / googletest is no longer a mandatory dependency. It's now included via find_package and the test target is not built if it's not found.
• Remove hacks for including headers and linking libraries. cmake and pkg-config may be used to get the right flags.
• Add .clang-format, it appears to be the style aready used.
• ci now fetches libextism with a github token to avoid rate limiting

[zshipko]

👍