[feature] make overriding git metadata embedding more flexible

[robomics]

More specifically, prefix all GIT_* CMake variables with HICTK_, so that projects importing hictk with e.g. add_subdirectory() who are also using https://github.com/andrew-hardin/cmake-git-version-tracking to embed git metadata in their applications can override git metadata for hictk (without affecting the metadata of their main project).