Load OAuth 2.0 token exchange credentials provider from config file

[UgnineSirdis]

Load OAuth 2.0 token exchange credentials provider from config file
Implement RFC part in Java SDK: https://github.com/ydb-platform/ydb-rfc/blob/main/cli_oauth2_token_exchange.md

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.30088% with 40 lines in your changes missing coverage. Please review.

Project coverage is 46.47%. Comparing base (2327294) to head (bf62f5a).

Files	Patch %	Lines
...ava/tech/ydb/auth/OAuth2TokenExchangeProvider.java	84.65%	9 Missing and 20 partials ⚠️
...src/main/java/tech/ydb/auth/OAuth2TokenSource.java	70.27%	7 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #296      +/-   ##
============================================
+ Coverage     45.87%   46.47%   +0.60%     
- Complexity     1638     1677      +39     
============================================
  Files           303      303              
  Lines         11874    12082     +208     
  Branches       1153     1207      +54     
============================================
+ Hits           5447     5615     +168     
- Misses         6013     6030      +17     
- Partials        414      437      +23     

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