Pull request: dhcpd: add purge, imp code

Updates #1691.

Squashed commit of the following:

commit 2ce6cc005d09ac7d63e4e575705d86e92046697f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jun 16 16:42:57 2021 +0300

    dhcp: imp code

commit 8f2bd7048d864e10aaed9e7338c0bbe852699a31
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jun 16 16:27:06 2021 +0300

    dhcpd: add purge, imp code

openapi/openapi.yaml

@@ -473,6 +473,21 @@
               'schema':
                 '$ref': '#/components/schemas/Error'
           'description': 'Not implemented (for example, on Windows).'
+  '/dhcp/reset_leases':
+    'post':
+      'tags':
+      - 'dhcp'
+      'operationId': 'dhcpResetLeases'
+      'summary': 'Reset DHCP leases'
+      'responses':
+        '200':
+          'description': 'OK.'
+        '501':
+          'content':
+            'application/json':
+              'schema':
+                '$ref': '#/components/schemas/Error'
+          'description': 'Not implemented (for example, on Windows).'
   '/filtering/status':
     'get':
       'tags':