Add diff option to dr config:export and config:import

Problem/Motivation

The config export and import commands behave like the corresponding drush commands without any options.
The --diff option displays what in the config changes rather than just the name.
This is useful for dr as well but out of scope of the first command implementation.

Steps to reproduce

run dr config:export or dr config:import

Proposed resolution

add --diff option to both commands.
There is a good example in config_checkpoint_tools

Remaining tasks

refactor common methods into a trait.

User interface changes

added --diff option to dr commands

Introduced terminology

none

API changes

none

Data model changes

none

Release notes snippet

dr commands config:export and config:import now accept --diff option.