Add parameter to corrosion_import_crate to return list of added targets

[temeddix]

This PR adds new IMPORTED_CRATES flag to corrosion_import_crate() to retrieve the list of imported crates in the current scope. Idea was from issue #218.

With this PR, this kind of operation is made possible.

# Part of CMAKE file
corrosion_import_crate(MANIFEST_PATH ../native/Cargo.toml IMPORTED_CRATES crates_list)
target_link_libraries(${BINARY_NAME} PRIVATE ${crates_list})

I checked that this works as intended in an actual Rust/C++ project.

[jschwe]

Thanks for the PR!