Add support for Apple framework builds

[Treata11]

Greetings,

Similar to the PR in libde265, I added the essential properties to the heif target to enable cross-compilation for iOS derived platforms.
Since libheif depends on libde265, the framework artifacts of libde265 must exist on local machine in order to cross compile libheif for a desired Apple platform.

The following commands could be used to generate the frameworks for an iOS target:

cmake -S. -B build -DBUILD_FRAMEWORK=TRUE -DCMAKE_SYSTEM_NAME=iOS -DWITH_EXAMPLES=OFF

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

Note that WITH_EXAMPLES=OFF has to be off since I didn't find it necessary to modify the properties of Examples.

[farindk]

Thanks @Treata11 for the PR and @kmilos for the review.