Add byte display option in Network Monitor plasmoid

Summary:
Add the option to select between byte and bit display in Network Monitor plasmoid.
Bug: 383019

{F7826928}

Reviewers: #vdg, #plasma, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26124