Use shared frameworks with LTO in the React Native iOS prebuild

[fealebenpae]

In #3649 we moved to prebuilding the RN iOS source code and shipping it as a set of static libraries packaged in an xcframework. That has a downside in that the static libraries are too big. We should instead build a shared framework, which is smaller, and allows us to use link-time optimization to boot.

It's definitely possible to build a shared framework for iOS from CMake, that's what we do over in the .NET SDK. The main difference is that we don't have to ship headers in the framework in .NET, but here our framework needs to ship a couple of header files, and I wasn't able to figure out the CMake incantation to get it to copy these headers in the final framework bundle.

Once CMake builds frameworks (with the proper headers) for device and simulator xcodebuild -create-xcframework can stitch them together just like it does the static libraries.

[kraenhansen]

I wonder where this stands with #6650 merged. When we're combining library files using libtool

realm-js/packages/realm/src/scripts/build/xcode.ts

Line 114 in 13e70d7

const { status } = spawnSync("xcrun", ["libtool", "-static", "-D", "-o", outputPath, ...inputPaths], {

we use -static instead of -dynamic and I wonder if that would address this issue?