CMake: add PROJ_DB_CACHE_DIR

[mwtoews]

This re-implements PROJ_DB_CACHE_DIR to CMake, which is similar to #1446 which was lost when Autotools were dropped.

Local tests on Ubuntu 20.04 work nicely, using something like this:

mkdir /tmp/proj_cache
mkdir build && cd build
cmake -DPROJ_DB_CACHE_DIR=/tmp/proj_cache ..
make -j7
make clean
make -j7

This option works nicely with USE_CCACHE (i.e. -DUSE_CCACHE=ON) to create subsequent builds within a few seconds.

Integration with CI will happen later.

Closes #3710