firewall_policy (2020-03-01): `FirewallPolicyNatRuleActionType` is defined as `DNAT` while service returns `Dnat`

(At least) In version 2020-03-01, the `FirewallPolicyNatRuleActionType` enum allows accept `DNAT`, while the response of that attribute is `Dnat`. This causes some unnecessary diff for IaC tool (such as terraform).

To make the API idempotent, suggest to return the `FirewallPolicyNatRuleActionType` unchanged.

Locate the version 2020-03-01 firewall_policy specification and inspect the FirewallPolicyNatRuleActionType enum. Compare the accepted DNAT value with the service's Dnat response, then verify that the specification preserves the service casing and avoids an IaC diff.

Written by the indexing model from the issue text.