This patch builds on the parent commit, but changes the API
to use a target property.

Since this approach requires CMake >= 3.19, the rust2cpp test that received features
was copied into the new features test and reverted from the old test, so that
it can still be run with older cmake versions.

* Make sure to use a space separator for the cargo command line
* Join the target property list to the expected comma delimiter for the generator

And $<IF> without the else part can be rewritten as plain $<>

Reference the second function that's guarded by the second feature, otherwise we don't
know if both were passed.

This restores what 79e498d added initially,
the most convenience API for the common case of importing just one crate.

The target property is empty, if it does not exist. If it is not empty, then
map it to the correct value (--foo) or the empty string.