feature: implement --dedup-plugin-configs flag for dump command

_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. The scope here is to automate the process for our users:

How can we do this?
After decK exports Kong's configuration but before it writes the configuration to file, deck analyzes if there are multiple plugins in the configuration that share identical configuration. If so, it will de-duplicate those configuration by putting them under _plugin_configs section and then add a reference to them wherever that configuration is in use.

Start at the dump command's export-to-file path: the requested analysis runs after Kong configuration export and before writing. Define completion by finding repeated identical plugin configurations, placing shared entries under _plugin_configs, and replacing each use with a reference.