Use shared key auth for custom blob resource so we don't need RBAC

[thomas11]

This PR resolves #3709. Writing a blob failed with 403 Forbidden using the new azcore backend but succeeded using the old backend.

Both backends are based on Azure authentication tokens. However, for blob operations, the old implementation used the token only to get a shared access key for the storage account, then used the key for all other operations. The new implementation attempted to use the auth token directly for blob operations like create.

Using the token directly works if the user/principal has the correct RBAC role, which is "Storage Blob Data Owner" (and possibly others). That's how our CI test suite passed: our service principal had that role assigned.

RBAC is actually the preferred way of accessing Azure storage. But not all users of this provider have the required roles configured, and we cannot regress scenarios that used to work in the v2 major version, so we need to switch to using access keys even in the new backend.

Here is a full CI test suite run for this branch.
Here's the one with autorest enabled.

[github-actions]

Does the PR have any schema changes?

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

[codecov]

Codecov Report

Attention: Patch coverage is 67.70833% with 31 lines in your changes missing coverage. Please review.

Project coverage is 58.97%. Comparing base (93fac42) to head (7a9d7f8).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...urces/customresources/custom_storage_azidentity.go	67.36%	24 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3719      +/-   ##
==========================================
+ Coverage   58.88%   58.97%   +0.08%     
==========================================
  Files          74       74              
  Lines       11738    11811      +73     
==========================================
+ Hits         6912     6965      +53     
- Misses       4288     4302      +14     
- Partials      538      544       +6     

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[pulumi-bot]

This PR has been shipped in release v2.73.1.