Initial sketch of using azcore client

[mikhailshilkov]

I haven't wired it anywhere, so it's no functional change, but I was playing with azcore to see if it solves #903 (it does not) and implemented the minimal version of CRUD. It worked for resource groups/vnets/subnets, but obviously lots of work to do to make it universal and then gradually introduce it to users.

First step towards #1464

@thomas11 @danielrbradley @mjeffryes Do you folks think it's worth pursuing?

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[thomas11]

Nice! I think it's definitely worth pursuing since we have to do #1464 at some point. The question is, as always, prioritization, i.e., when to do it.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 108 lines in your changes are missing coverage. Please review.

Project coverage is 60.26%. Comparing base (1586bd5) to head (fc82f2c).
Report is 1 commits behind head on master.

Files	Patch %	Lines
provider/pkg/azure/client_azcore.go	0.00%	108 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3128      +/-   ##
==========================================
- Coverage   60.83%   60.26%   -0.58%     
==========================================
  Files          71       72       +1     
  Lines       11353    11461     +108     
==========================================
  Hits         6907     6907              
- Misses       3883     3991     +108     
  Partials      563      563              

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