Add sub-module which subscribes to the Drupal 8.8 config storage transformation events

Problem/Motivation

config_merge comes with the config_merge_filter module that hooks up config_merge to the snapshot storage via config_filter

Since Drupal 8.8 there are the config storage transformation events, so we could do the same without depending on config_filter.

Proposed resolution

Add config_merge_event module with one event subscriber service, subscribing to the config import transformation event, merging the snapshot config like config_merge_filter does via a config filter plugin.

Remaining tasks

Do it.

User interface changes

new sub module

API changes

none

Data model changes

none