plugin/azure: add refresh directive (#8614)

Brings azure to parity with route53, which already exposes a refresh
directive to control how often zones are re-synced. azure currently
hardcodes this to 1 minute (Run calls h.run(ctx, time.Minute)) with no
way to change it, even though the internal run() method already takes
an arbitrary interval parameter - the plumbing exists, it just wasn't
exposed as a Corefile option.

Adds a refresh field to the Azure struct, threads it through New(), and
has Run() use h.refresh instead of the hardcoded value. Parses the
refresh directive in parse() mirroring route53's exact validation
(accepts a plain number as seconds or a duration string, must be
greater than 0).

Extends TestSetup with refresh coverage (valid duration, valid plain
number, missing arg, non-numeric, zero, negative). Verified it fails
without the setup.go change (unknown property error) and passes with
it.

Signed-off-by: stackedbyaradhya <aradhyaj736@gmail.com>

plugin/azure/sync_test.go

@@ -56,7 +56,8 @@ func newTestAzure(t *testing.T, private bool, handle azureAPIHandler) (*Azure, *
 	}
 	h, err := New(context.Background(), publicClient, privateClient,
 		map[string][]string{"rg": {"healthy.example", "missing.example"}},
-		map[string]string{"rghealthy.example": access, "rgmissing.example": access})
+		map[string]string{"rghealthy.example": access, "rgmissing.example": access},
+		time.Minute)
 	if err != nil {
 		t.Fatal(err)
 	}