feat: support externalId for ram role arn

[yndu13]

You need to complete

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

[codecov-commenter]

Codecov Report

Merging #51 (be6e710) into master (844da20) will increase coverage by 0.90%.
The diff coverage is 83.33%.

❗ Current head be6e710 differs from pull request most recent head 41440e8. Consider uploading reports for the commit 41440e8 to get more accurate results

@@             Coverage Diff              @@
##             master      #51      +/-   ##
============================================
+ Coverage     84.42%   85.33%   +0.90%     
- Complexity      482      501      +19     
============================================
  Files            44       44              
  Lines          1740     1752      +12     
  Branches        111      112       +1     
============================================
+ Hits           1469     1495      +26     
+ Misses          234      221      -13     
+ Partials         37       36       -1     

Flag	Coverage Δ
unittest	85.33% <83.33%> (+0.90%)	⬆️

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Impacted Files	Coverage Δ
...ain/java/com/aliyun/credentials/models/Config.java	42.85% <33.33%> (+25.87%)	⬆️
src/main/java/com/aliyun/credentials/Client.java	70.23% <100.00%> (+0.35%)	⬆️
...entials/provider/RamRoleArnCredentialProvider.java	98.12% <100.00%> (+1.41%)	⬆️