Additional CLI options for length-prefixed message output and endianness

[salsferrazza]

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.