Add real credentials provider

[JacksonTian]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.24953% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.53%. Comparing base (ba4db56) to head (b67adee).

Files	Patch %	Lines
sdk/auth/rpc_signature_composer.go	81.25%	2 Missing and 1 partial ⚠️
sdk/client.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #634      +/-   ##
==========================================
+ Coverage   93.50%   94.53%   +1.03%     
==========================================
  Files          48       50       +2     
  Lines        2540     3020     +480     
==========================================
+ Hits         2375     2855     +480     
  Misses        107      107              
  Partials       58       58              

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