improve ram role arn credentials provider

[JacksonTian]

You need to complete

• unit tests and/or feature tests
• documentation is changed or added

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 62.71186% with 22 lines in your changes missing coverage. Please review.

Project coverage is 93.15%. Comparing base (09e2c68) to head (a8ec063).

Files	Patch %	Lines
credentials/internal/providers/ram_role_arn.go	48.27%	14 Missing and 1 partial ⚠️
credentials/credential.go	76.66%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   94.60%   93.15%   -1.45%     
==========================================
  Files          26       26              
  Lines        1631     1651      +20     
==========================================
- Hits         1543     1538       -5     
- Misses         85      107      +22     
- Partials        3        6       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[JacksonTian]

测试覆盖率下降主要因为代理部分。