[mlir][IR][NFC] Move CastOpInterface helpers to mlir/Interfaces

These helpers should not be part of the IR build unit.

The interface is now implemented on `builtin.unrealized_conversion_cast` with an external model.

Also rename the CastOpInterfaces Bazel target name to CastInterfaces to be consistent with the CMake target name.

Differential Revision: https://reviews.llvm.org/D146972