[mlir][gpu] Add the GPU offloading handler attribute interface.

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

Commit message:
This patch adds the `OffloadingTranslationAttrTrait` trait and the
`GPUOffloadingLLVMTranslationAttrInterface` attribute interface.

The interface indicates that an attribute is a GPU offloading translation
attribute. These kinds of attributes must implement an interface for handling
the translation of GPU offloading operations like `gpu.binary` & `gpu.launch_func`.

The interface is meant to be used as an opaque interface for embedding GPU
binaries into LLVM modules and launching GPU kernels.

Depends on D154129

Reviewed By: krzysz00

Differential Revision: https://reviews.llvm.org/D154108