refactor: Add auth_config mock

[helayoty]

This PR includes the following changes:

1. To be able to write unit tests for the auth package, we need to mock the azidentity and autorest libraries.

   In order to achieve that, we introduce a new interface, ConfigInterface that will implement:

   • GetMSICredential
   • GetSPCredential
   • GetAuthorizer

2. And to make the code more organized, we split the config away from Authentication into two separate files; auth_config and authentication.

3. Update the deprecated io/ioutil and replace it with io.

[codecov-commenter]

Codecov Report

Merging #528 (93557f2) into master (7e523b0) will decrease coverage by 0.37%.
The diff coverage is 57.74%.

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@@            Coverage Diff             @@
##           master     #528      +/-   ##
==========================================
- Coverage   61.36%   60.99%   -0.37%     
==========================================
  Files          15       17       +2     
  Lines        2490     2505      +15     
==========================================
  Hits         1528     1528              
- Misses        850      865      +15     
  Partials      112      112              

Impacted Files	Coverage Δ
pkg/auth/mock_auth_config.go	0.00% <0.00%> (ø)
pkg/auth/authentication.go	72.22% <72.22%> (ø)
pkg/auth/auth_config.go	61.53% <100.00%> (ø)

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