Add WritableFile::Tell, make tf.GFile.tell call it.

Currently, tf.GFile.tell fails if the file was opened for writing. This is
undesirable behavior.

This change adds WritableFile::Tell and has tf.GFile.tell delegate to that in
the case of a writable file.

The Tell method returns the write position in the given writable file. This
method is intended to function like the POSIX ftell method, but any
WritableFile implementation may choose to return errors::Unimplemented (the
default behavior).

Implementations are provided for posix, HDFS, gzip, GCS and some others.

PiperOrigin-RevId: 226056765