Virtual network gateway changes for dns forwarding

[absrivas12]

This pull request introduces a new boolean flag enableDnsForwarding on the VirtualNetworkGateway resource. The flag can be set to true or false using the PUT operation on VirtualNetworkGateway. If the flag is not specified then it is a no op on the resource provider.
The pull request also adds a string property called inboundDnsForwardingEndpoint which is a readonly property, set automatically when PUT with enableDnsForwarding set to true succeeds.

When a PUT operation is done where enableDnsForwarding is true, it enables the DNS forwarding functionality on the virtual network gateway, and the gateway can be used as a dns forwarder. After successful completion of operation the users will be able to send DNS requests to the gateway at the IP address specified by the inboundDnsForwardingEndpoint property. This is a readonly property whose value is automatically set by the resource provider on the VirtualNetworkGateway resource.

Users can also disable dns forwarding functionality on the gateway by using a PUT where enableDnsForwarding is set to false.

Latest improvements:

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[msftclas]

All CLA requirements met.

[AutorestCI]

Automation for azure-sdk-for-python

Encountered a Subprocess error: (azure-sdk-for-python)

Command: ['/usr/local/bin/autorest', '/tmp/tmppp8df_i8/rest/specification/network/resource-manager/readme.md', '--keep-version-file', '--multiapi', '--no-async', '--python', '--python-mode=update', '--python-sdks-folder=/tmp/tmppp8df_i8/sdk/sdk', '--use=@microsoft.azure/autorest.python@4.0.70', '--version=preview']
Finished with return code -6
and output:

AutoRest code generation utility [version: 2.0.4283; node: v10.15.3]
(C) 2018 Microsoft Corporation.
https://aka.ms/autorest
   Loading AutoRest core      '/root/.autorest/@microsoft.azure_autorest-core@2.0.4399/node_modules/@microsoft.azure/autorest-core/dist' (2.0.4399)
   Loading AutoRest extension '@microsoft.azure/autorest.python' (4.0.70->4.0.70)
   Loading AutoRest extension '@microsoft.azure/autorest.modeler' (2.3.44->2.3.44)
Processing batch task - {"tag":"package-2019-06"} .

<--- Last few GCs --->

[8410:0x2937cc0]   145567 ms: Mark-sweep 1381.2 (1427.3) -> 1381.2 (1427.8) MB, 2614.3 / 0.0 ms  (average mu = 0.095, current mu = 0.001) allocation failure GC in old space requested
[8410:0x2937cc0]   148282 ms: Mark-sweep 1381.9 (1427.8) -> 1381.8 (1428.3) MB, 2711.7 / 0.0 ms  (average mu = 0.051, current mu = 0.001) allocation failure GC in old space requested


<--- JS stacktrace --->

==== JS stack trace =========================================

    0: ExitFrame [pc: 0x1030b675be1d]
Security context: 0x11640e21e6e9 <JSObject>
    1: newScalar [0xd7fea07b841] [/node_modules/yaml-ast-parser/dist/src/yamlAST.js:~38] [pc=0x1030b6fc3276](this=0x25a38732dc71 <Object map = 0x1ac7649c4e61>,v=0x3ca8e48026f1 <undefined>)
    2: arguments adaptor frame: 0->1
    3: readDoubleQuotedScalar(aka readDoubleQuotedScalar) [0x25a38732e571] [/node_modules/yaml-ast-parser/dist/src/loader.js:~530] [pc...

FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
 1: 0x8dc510 node::Abort() [node]
 2: 0x8dc55c  [node]
 3: 0xad9b5e v8::Utils::ReportOOMFailure(v8::internal::Isolate*, char const*, bool) [node]
 4: 0xad9d94 v8::internal::V8::FatalProcessOutOfMemory(v8::internal::Isolate*, char const*, bool) [node]
 5: 0xec7bf2  [node]
 6: 0xec7cf8 v8::internal::Heap::CheckIneffectiveMarkCompact(unsigned long, double) [node]
 7: 0xed3dd2 v8::internal::Heap::PerformGarbageCollection(v8::internal::GarbageCollector, v8::GCCallbackFlags) [node]
 8: 0xed4704 v8::internal::Heap::CollectGarbage(v8::internal::AllocationSpace, v8::internal::GarbageCollectionReason, v8::GCCallbackFlags) [node]
 9: 0xed7371 v8::internal::Heap::AllocateRawWithRetryOrFail(int, v8::internal::AllocationSpace, v8::internal::AllocationAlignment) [node]
10: 0xea07f4 v8::internal::Factory::NewFillerObject(int, bool, v8::internal::AllocationSpace) [node]
11: 0x11402db v8::internal::Runtime_AllocateInTargetSpace(int, v8::internal::Object**, v8::internal::Isolate*) [node]
12: 0x1030b675be1d

[azuresdkci]

Can one of the admins verify this patch?

[AutorestCI]

Automation for azure-sdk-for-go

Nothing to generate for azure-sdk-for-go

[anton-evseev]

LGTM except one thing

[anton-evseev]

Please also add new properties in example response (since without them it's not acquirate)

[anton-evseev]

@tjprescott assigning this to you since Catherine Zhu said you're one of those who would cover for Chinese team during their long holiday. Could you please merge this PR?