Fix account credentials

[DaanRademaker]

• Create new function with account client which is used for account level api's
• Refactor credential code to use databricks-sdk
• Write unittest for python for credentials code
• Write jest tests for credentials

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (8ea9233) 63.15% compared to head (1cce93b) 64.31%.

Files	Patch %	Lines
aws-lambda/src/databricks_cdk/utils.py	40.00%	3 Missing ⚠️
aws-lambda/src/databricks_cdk/resources/handler.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1064      +/-   ##
==========================================
+ Coverage   63.15%   64.31%   +1.16%     
==========================================
  Files          38       38              
  Lines        1919     1903      -16     
==========================================
+ Hits         1212     1224      +12     
+ Misses        707      679      -28     

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