BearerToken credential provider refactor

[doublnt]

You need to complete

• [x] unit tests and/or feature tests
• [ ] documentation is changed or added

[codecov-io]

Codecov Report

Merging #227 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
+ Coverage   96.54%   96.55%   +0.01%     
==========================================
  Files          62       63       +1     
  Lines        3099     3109      +10     
  Branches      296      297       +1     
==========================================
+ Hits         2992     3002      +10     
  Misses         68       68              
  Partials       39       39

Flag	Coverage Δ
#unittest	96.55% <100%> (+0.01%)	:arrow_up:

Impacted Files	Coverage Δ
...-sdk-core/Auth/Credential/BearerTokenCredential.cs	100% <100%> (ø)	:arrow_up:
...ore/Auth/Provider/BearerTokenCredentialProvider.cs	100% <100%> (ø)
aliyun-net-sdk-core/Http/HttpResponse.cs	93.84% <0%> (-2.31%)	:arrow_down:
...uth/Provider/InstanceProfileCredentialsProvider.cs	100% <0%> (+8.33%)	:arrow_up:

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