Adding Discord-only TurboModule support

[Flewp]

In This PR..

Adding Support for building only Discord TurboModules.

How It's Done

We accomplish in two parts (TL;DRs in bold)

1. Adding an option in ReactPlugin.kt to codegen only modules with a package name starting with com.discord. Codegen is the "third pillar" of "The New Architecture". It allows us to define the schema of what's communicated through the JSI, and have the Makefiles/Java/etc. generated for us, to the point where all we need to do is include and implement their generated interface on the native side, and call the generated interface from the JS side. The vanilla way to codegen is by running the generateCodegenArtifactsFromSchema gradle task with newArchEnabled=true, normally this task is added as part of the build when newArchEnabled=true, so it's not normally run standalone, though it can be. This task is problematic because some of our third party dependencies, like react-native-reanimated or react-native-gesture-handler have codegen support, and this task will codegen every module that has codegen support. If we run codegen, then run a build with newArchEnabled=false, it creates build issues with these third party dependencies because their generated schemas share the same class name as their legacy-module-schema equivalents, thus we get class name duplicate errors. The fix for this is to add in a new project level gradle option: onlyDiscordTurboModulesEnabled. This creates an extra condition during the codegen process to only codegen modules that begin with com.discord, we can then run (at the react-native/ReactAndroid level)

ORG_GRADLE_PROJECT_newArchEnabled=true ORG_GRADLE_PROJECT_onlyDiscordTurboModulesEnabled=true ./gradlew generateCodegenArtifactsFromSchema

This runs the task with both newArchEnabled and onlyDiscordTurboModulesEnabled set to true, codegen-ing only Discord TurboModules.

2. Adding extra options to the generated CMakeLists.txt to support libraries/headers needed to compile generated code with the JSI. The JSI (javascript interface) is React Native's "new way" of communicating between javascript and native code synchronously, it's written in C++. The JSI is the alternative method of the "old way" of communicating between JS/native, via the JSON bridge asynchronously. When a module is codegen'd, it generates a CMakeLists.txt describing how to compile the generated .h and .cpp files, which are the JSI layer of the TurboModule. In order to compile against the JSI layer, we need a handful of libraries (*.so) and their associated headers. These resources are assumed to be available, but they aren't when compiling with newArchEnabled=false. These new options allow us to pass in a couple extra CMake options: PREBUILT_CMAKE, which is a .cmake file that includes library definitions for the needed JSI *.sos. EXTRA_INCLUDE_DIRECTORIES, which allows us to define an extra set of header directories, so the compiled source knows where <fbjni/fbjni.h> and all of the other includes are.

Assumptions

For the moment, we must build React Native from source if we're building out these Discord-only-TurboModules. We can fix this in the future, but there's two versions of react-native-codegen in this repository. One in packages/ for when building React Native from source, and one provided via yarn, which'll eventually end up in node_modules, for when providing a pre-built React Native artifact. This PR only includes the impl for the build-from-source approach while we stand up the initial Discord TurboModules. A future PR will provide the impl for the prebuilt approach.