Add support for Apple-Framework builds

[Treata11]

Greetings,

I introduced the required properties in CMakeLists.txt to enable cross-compilation for iOS-derived platforms as explained in the CMake documentation.

Binaries of brotlicommon.framework, brotlidec.framework & brotlienc.framework targets will be generated in the build directory with the following commands:

cmake -S. -B build -DBUILD_FRAMEWORK=TRUE -DCMAKE_SYSTEM_NAME=iOS -DBROTLI_BUNDLED_MODE=ON

cmake --build build --target install --config Release

It's also worth noting that I included the BROTLI_PUBLIC_HDRS files for the targets above to ensure they are included in the built framework bundles.

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[Treata11]

Addresses issue #1245

[Treata11]

@eustas, Changes above are sufficient to build for iOS-derived platforms & their simulators.
Separate framework bundles are generated for each target:
(brotlicommon.framework, brotlidec.framework & brotlienc.framework).