Add option to avoid edge cache clears in maintenance mode

Problem/Motivation

We have twice weekly deploys that are clearing the edge caches because our deploy process is to always run a `drush cache:rebuild`. It's hard to deploy without clearing the caches. There are just too many cases where a rebuild is required.

We do all our deploys while in maintenance mode. so we'd like the maintenance mode cache clears to only clear the Drupal caches but not the edge caches. If an edge cache clear is required, we'll do a separate cache rebuild while not in maintenance mode. While I can do this with custom code, it seems like a worthwhile feature for others.

Proposed resolution

Add an option to this module that prevents edge cache clears while in maintenance mode.

Data model changes

A new config key was added. If it's not set, nothing changes. If it is set, then cache rebuilds in maintenance mode won't clear the edge cache.