chore(housekeeping): Create helpers/change-detection.sh (#2610)

* chore: Extract change-detection method to it's own helper

This doesn't really belong in `helpers/ssl.sh`. Moving to it's own helper script.

* chore: Co-locate related change-detection method from container startup

It seems relevant to migrate the related support during startup for the change detection feature into this helper.

I opted to move the call from `start-mailserver.sh` into the `_setup` call at the end for a more explicit/visible location.

* chore: Move `CHKSUM_FILE` into `helpers/change-detection.sh`

It belongs there, not in `helpers/index.sh`.

* chore: Revise inline documentation

* tests(fix): Ensure correct functionality

Presently `test/test_helper.bats` is using it's own  `CHKSUM_FILE` instead of sourcing the var for the filepath.

`test_helper/common.bash` was calling a method to check for changes, but this helper may not correctly detect letsencrypt related changes as these are not ENV rely on, but global vars handled by `helpers/dns.sh`, so that should be run first like it is for `check-for-changes.sh`.

* tests(chore): Use `CHKSUM_FILE` var from helper

* chore: `addmailuser` should use `CHKSUM_FILE` var

* chore: Update `check-for-changes.sh` log message with correct path

target/scripts/start-mailserver.sh

@@ -245,9 +245,6 @@ function _register_functions
   _register_setup_function '_setup_logwatch'
   _register_setup_function '_setup_user_patches'
 
-  # needs to come last as configuration files are modified in-place
-  _register_setup_function '_setup_chksum_file'
-
   # ? >> Fixes
 
   _register_fix_function '_fix_var_mail_permissions'