provider/azurerm: network interfaces and public IP support.

[aznashwan]

❗ Not to be merged yet! ❗

Considering the two resource go hand in hand so well together, this PR aims to bring support for both ARM network interfaces and public IPs.

@phinze @jen20: do please glance over the observations I'll leave on the diff; some of them address your proposal from back off load balancer PR (#4343).

PS: for general continuity; this PR was branched from the local network gateways PR (#4342), so please feel free to ignore those files.

[aznashwan]

@phinze: concerning your suggestions to move some of these sub-resources to the top-level: in this particular case, and I'd suspect many more, Creating a network interface without defining at least one such IP configuration leads to a 400.

Also, as I'll point out in the test config; most of these sub-resources have id fields as well and can/must be used in conjunction with other resource/sub-resources.
As such; we may be able to get away with separating some resources; but most will likely turn out impractical to separate.

[aznashwan]

NOTE for when we add subnets...

[aznashwan]

See you on the other side boys 😄

[jen20]

Hi all, @stack72 ended up implementing these resources #4565 and #4598, which were subsequently merged, so I'll close this pull request now. Thanks for your work @aznashwan!

[ghost]

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