[ty] Add basic support for overloads in `ParamSpec` (#21946)
## Summary
fixes: https://github.com/astral-sh/ty/issues/1838
This PR adds basic support for overloaded function when used to
specialize a `ParamSpec` type variable.
Following cases are still remaining:
1. Updating the specialization with the matching overload after the
paramspec sub-call logic
2. Updating the specialization with the matching overload using the
return type
Both of these cases are present in the mdtest file.
## Test Plan
Update mdtest with new cases.
