Support file-to-file translation in mlir-translate

Existing translations are either from MLIR or to MLIR. To support
cases like round-tripping some external format via MLIR, one must
chain two mlir-translate invocations together using pipes. This
can be problematic to support -split-input-file in mlir-translate
given that it won't work across pipes.

Motivated by the above, this CL adds another translation category
that allows file to file. This gives users more freedom.

PiperOrigin-RevId: 269636438