feat: implement --dedup-plugin-configs flag for dump

[GGabriele]

_plugin_configs feature of decK allows users to abstract out configuration of plugins that are used repeatedly and then reference them in services/routes/consumers. This helps with de-duplication of configuration code. While users would like to take advantage of this, most of them cannot because de-duplicating the configuration in the first place is a cumbersome and an error-prone process.

https://github.com/Kong/deck/issues/341 (doc)

This implements such flag and feature for the dump command.

Test: sync this file first (before.yaml.txt), having some shared plugins across different consumers, routes and services.

$ deck sync
<--omitted->

$ deck dump --yes --dedup-plugin-configs

$ deck sync
Summary:
  Created: 0
  Updated: 0
  Deleted: 0

Result: after.yaml.txt