CMake ICU options

There are a few CMake build options for ICU currently:

- `NO_ICU` disables building with ICU support
- `SYSTEM_ICU` triggers default ICU link libraries to be used
- `EMBED_ICU` download ICU sources, then build and use them during build
- `ICU_INCLUDE_PATH` custom ICU installation (just the include path)

There is some detection of the library being present, in main CMake for custom ICU path and in PAL for one of the header files.

It might be beneficial to refactor ICU support around detecting the library from main CMakeLists.txt:

- `WITH_ICU` - enable ICU, fail if library not found; default to ON if the library is present, and OFF otherwise
- `ICU_PREFIX` to point it to the location of custom of ICU installation, implies `WITH_ICU`
- `EMBED_ICU` download and build ICU, implies `WITH_ICU`