Added network watcher APIs

[irrogozh]

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[amarzavery]

@jhendrixMSFT - This PR was reviewed in the private repo. Hence no need to review this again in the public repo.

[amarzavery]

There have been multiple PRs in the Network RP space:

• recently closed
  • BGP Routes for VnetGateway @lamchester merged 3 days ago
    • .net sdk PR open
  • RouteFilter @dihan0604 merged 7 days ago, .net sdk PR still in review
    • .net sdk PR 2806 merged
• open
  • ApplicationGateway @jobatzil
  • the current PR NetworkWatcher @irrogozh

I know for sure that RouteFilter has some breaking changes to the signature of the "ExpressRouteCircuit" constructor. Hence, it is very important that all the owners of these sub resource types in the NRP space co-ordinate with each other and release one version of an sdk (client) per language (if possible), rather than publishing multiple versions in a very short span.

@DeepakRajendranMsft - Please take a look into this.

/cc @salameer @kirthik @johanste @lmazuel @cormacpayne @markcowl

[jhendrixMSFT]

Nit, spacing.

[AutorestCI]

No modification for NodeJS

[AutorestCI]

Azure/azure-sdk-for-ruby#681

[AutorestCI]

Azure/azure-sdk-for-python#989