jobspec: add diff for Task Group Scaling block

[nvanthao]

Without this diff nomad job plan won't report changes in scaling block.

Before:

$ nomad job plan job.hcl
+/- Job: "foo"
+/- Task Group: "foo" (1 create, 1 in-place update)
  +/- Count: "1" => "2" (forces create)
      Task: "foo"

After:

nomad job plan job.hcl
+/- Job: "foo"
+/- Task Group: "foo" (1 create, 1 in-place update)
  +/- Count: "1" => "2" (forces create)
  +   Scaling {
      + Enabled: "true"
      + Max:     "10"
      + Min:     "2"
      + Policy {
        + check[0][active_connections][0][query]:                                "scalar(open_connections_example_cache)"
        + check[0][active_connections][0][source]:                               "prometheus"
        + check[0][active_connections][0][strategy][0][target-value][0][target]: "20"
        + cooldown:                                                              "1m"
        + evaluation_interval:                                                   "5s"
        }
      }
      Task: "foo"

This PR is only for diff in TaskGroup Scaling block and not diff in Task Scaling block.

[lgfa29]

Thank you for the fix @nvanthao 😄

[nvanthao]

many thanks @lgfa29!