[AzureFirewall] NAT and FQDN Tags

[fanymanea]

Description

NAT Rule Collection

• A new type of Azure Firewall rule collection which allows users to perform DNAT on their packets.
• Each collection contains a list of NatRules, each rule specifying its own criteria for DNAT

FQDN Tag

• A way for users to specify services (e.g. AzureBackup, WindowsUpdate) which are allowed/denied through the firewall (instead of specifying a list of FQDNs - which they may not even know)
• FqdnTags cannot be used in the same rule with TargetFqdns

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • the changes have gone through a cmdlet design review
  • the cmdlet markdown files were generated using the platyPS module

[cormacpayne]

@azuresdkci add to whitelist

[praries880]

Retrying CI because of test failure in the last one that do not seem related to the PT at all.

[praries880]

@fanymanea Kindly take care of merge conflicts

[praries880]

@azuresdkci add to whitelist

[maddieclayton]

Will need to retarget this at release-2018-09-17 as soon as Mikhail's PR gets merged there.