Additional CLI options for length-prefixed message output and endianness

`LengthDelimitedOutputManager` takes `--prefix_length` to determine the size in bytes of the length prefix to output, but endianness is currently hard-coded. Additionally, the prefix length being read by `LengthDelimitedFileMessageSource` should be separately optioned from the prefix length written out. Hence `--source_prefix_length` should be an option for `LengthDelimitedFileMessageSource` and `--output_prefix_length` should be come an option instrumenting `LengthDelimitedOutputManager`.