GcsFileSystem should validate directories and file names for \n chars

Newlines are not valid characters in GCS object names ([https://cloud.google.com/storage/docs/naming](https://cloud.google.com/storage/docs/naming)), but we don't currently validate for this.  This leads to confusing errors if users accidentally include a trailing newline char on a string being passed into an IO.

The particular place where this came up was with FileIO using AvroIO.sink, where the directory passed to to() had a trailing newline, but it looks like this will be a problem generally with FileIO **** GCS. 

Imported from Jira [BEAM-7932](https://issues.apache.org/jira/browse/BEAM-7932). Original Jira may contain additional context.
Reported by: millsd@google.com.