c.o: improve monitoring for ldapcivi

Add a monitoring check for the REST endpoint on civicrm.org.

We had an issue last night where the LDAP service crashed because of a "composer update" that had been run automatically (#950 (closed)) on civicrm.org (and the update failed).

While we're going to remove those auto-updates, the LDAP service didn't throw any monitoring notifications.

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