YDB: specify IAM endpoint when using SA credentials

[vitalyisaev2]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 12.12121% with 29 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@a93a3b2). Learn more about missing BASE report.
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
app/server/config/config.go	0.00%	18 Missing and 2 partials ⚠️
.../server/datasource/rdbms/ydb/connection_manager.go	10.00%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #223   +/-   ##
=======================================
  Coverage        ?   63.06%           
=======================================
  Files           ?      115           
  Lines           ?     8034           
  Branches        ?        0           
=======================================
  Hits            ?     5067           
  Misses          ?     2626           
  Partials        ?      341           

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