FileSystems ReadableFileCoder should support Metadata#lastModifiedMillis()

Currently, there is a separated coder for Metadata which cares about `Metadata#lastModifiedMillis()`.
The issue is that the `ReadableFileCoder` always uses the default `Metadata` coder.
We need to create a similar coder for `ReadableFileCoder`: ``ReadableFileCoderV2` which just use the corresponding `MetadataCoderV2`.

Without doing so `lastModifiedMillis` will remain zero.

There is also no way to create custom coder in our side since `FileIO.ReadableFile` has no public method to initiate new instance of it.