add #[intertrait(crate = path::to::intertrait)] attribute support

this enables to integrate `intertrait` more easily in other libraries.

The issue names only the #[intertrait(crate = path::to::intertrait)] attribute and does not identify files or tests. Start by locating the crate's attribute or procedural-macro entry point and existing intertrait integration, then determine how the path should be resolved. Done means another library can use the attribute successfully and the relevant tests pass.

Written by the indexing model from the issue text.