Add separate options for `--input_prefix_length` and `--output_prefix_length`

`LengthDelimitedOutputManager` takes the `--prefix_length` command line option to size the int representing the prefix length for a message sent to the output stream. However, `LengthDelimitedInputSource` also uses the `--prefix_length` option for reading input streams that are length-prefixed.

The two options are distinct and should be separately instrumented. The clash namely occurs in a relatively rare situation, for example you are reframing messages and either narrowing or widening the prefix length as part of the process.