Add support for anonymous Azure blob access

[agocke]

The current support for Azure blob storage can't deal with the situation where the blob store needs to be accessed anonymously. This PR adds the opendal http backend and an environment variable that can be used to enable Azure anonymous access. If the environment variable is set, sccache falls back to direct http access instead of the opendal authentication flow.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.01149% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.73%. Comparing base (1b480c9) to head (eb58d24).

Files with missing lines	Patch %	Lines
src/cache/azure.rs	66.66%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2610      +/-   ##
==========================================
- Coverage   72.74%   72.73%   -0.01%     
==========================================
  Files          68       68              
  Lines       36985    37070      +85     
==========================================
+ Hits        26903    26964      +61     
- Misses      10082    10106      +24     

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[sylvestre]

some jobs are failing

[agocke]

Thanks, investigating.

[agocke]

I believe this is ready for re review/testing