Filter unchanged config from gateway diff option

Customer expressed interest to see only changed config when using `diff`. Below is the comment from them

`Another question on the deck, is there a feature to only show the lines that changed  + or -  and not the entire config?
For plugins that have a lot of configs such as openid-connect , it is hard to see what changed.`

Suggest including a suppress option for the diff command

Start at the diff command entry point and inspect how its existing options select and render configuration. Add a suppress option that shows only changed lines, then verify that unchanged configuration is omitted while the diff output still identifies additions and removals.

Written by the indexing model from the issue text.