forget json output: added id's in snapshots within reasons object

In order to evaluate the keep reasons for snapshots, there should be also the id's
to compare it with snapshots within the keep object. (See also Issue #3117)

In order to avoid output parameters also changed function addJSONSnapshots to asJSONSnapshots

changelog/unreleased/pull-4737

@@ -0,0 +1,5 @@
+Enhancement: include snapshot id in reason field of forget JSON output
+
+The JSON output of the `forget` command now includes the `id` and `short_id` of a snapshot in the `reason` field.
+
+https://github.com/restic/restic/pull/4737