LOCAL_SYMLINK Option in CMake Macros

Add an option `LOCAL_SYMLINK` to the `rez_install_files` and `rez_install_dirs` macros to create symlinks.

When installing files, this will create a symlink to the original source file, instead of performing a copy.  This is useful for developing files that don't require a compilation step (and so don't need rebuilding) to enable a quicker iteration for the developer.  Example of such files are mel, otls and (somewhat unfortunately I think) python.

A central `rez release` overrides this behaviour - it is not possible to release a symlink.