Debugger: File IO utils + File dumping from DebugOps

* Add debug_urls to existing Debug Ops to allow specification of debug URLs.

* Support file debug URLs, such as "file:///tmp/tfdbg_dump_1", where the path specifies a directory in which the dump files will be generated for intermediate tensors during a Session::Run() call.
  For example, given that the node name is "foo/bar/node_a", the output slot index of the dumped tensor is 0, and the debug Op is "DebugIdentity", the full path of the dump file will be: "/tmp/tfdbg_dump_1/foo/bar/node_a_0_DebugIdentity_${WALL_TIME_US}", where WALL_TIME_US is the wall timestamp for when the dumped tensor is generated, in microseconds.

* The debug_urls list of strings can be contain multiple individual URLs, e.g., {"file:///tmp/dump_1", "file:///tmp/dump_2"}, in which case, each debug signal tensor will be dumped to all specified paths.

* Add C++ and Python unit tests.

Other related changes:

* Move debugger- (tfdbg-)related build targets from core/BUILD to core/debug/BUILD

Future change lists will implement GRPC debug URL targets.
Change: 128873549