Add bearertoken support

[doublnt]

You need to complete

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

[codecov-io]

Codecov Report

Merging #180 into master will increase coverage by 0.24%. The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   92.45%   92.69%   +0.24%     
==========================================
  Files          62       62              
  Lines        2914     2942      +28     
  Branches      275      280       +5     
==========================================
+ Hits         2694     2727      +33     
+ Misses        178      170       -8     
- Partials       42       45       +3

Flag	Coverage Δ
#unittest	92.69% <96.55%> (+0.24%)	:arrow_up:

Impacted Files	Coverage Δ
aliyun-net-sdk-core/Auth/RpcSignatureComposer.cs	100% <ø> (ø)	:arrow_up:
aliyun-net-sdk-core/Auth/Signer.cs	100% <100%> (ø)	:arrow_up:
aliyun-net-sdk-core/DefaultAcsClient.cs	84.38% <100%> (-0.14%)	:arrow_down:
aliyun-net-sdk-core/Auth/BearerTokenSigner.cs	100% <100%> (+100%)	:arrow_up:
aliyun-net-sdk-core/RoaAcsRequest.cs	99.09% <93.75%> (-0.91%)	:arrow_down:
aliyun-net-sdk-core/RpcAcsRequest.cs	99.1% <95.65%> (-0.9%)	:arrow_down:
...dk-core/Auth/InstanceProfileCredentialsProvider.cs	97.22% <0%> (-2.78%)	:arrow_down:
...-sdk-core/Auth/Credential/BearerTokenCredential.cs	100% <0%> (+23.07%)	:arrow_up:

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[AxiosLeo]

补一下UT吧