[mlir][gpu] Add GPU target attribute interface.

**For an explanation of these patches see D154153.**

Commit message:
This patch adds the `GPUTargetAttrInterface` attribute interface, this interface
is meant to be used as an opaque interface for serializing GPU modules into
binary strings.

Reviewed By: mehdi_amini, krzysz00

Differential Revision: https://reviews.llvm.org/D154104