Formatter: add a dump_format_ns to base Formatter

Formatter: add a dump_format_ns to base Formatter

`dump_format_ns` is a generic formatter to dump a simple format along
with a namespace. It is `dump_format` with an optional ns. This also
extends the XML formatter with this functionality. This allows creation
of xml tags with ns and a specified format. The JSON Format doesn't
define a ns, and here the functionality is exactly same as that of
`dump_format`

Signed-off-by: Abhishek Lekshmanan <abhishek.lekshmanan@ril.com>