New module with OAuth2 provider implementation

[alex268]

Fix #272

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.20059% with 40 lines in your changes missing coverage. Please review.

Project coverage is 45.71%. Comparing base (ef49df2) to head (e4d7f72).
Report is 6 commits behind head on master.

Files	Patch %	Lines
...in/java/tech/ydb/core/auth/BackgroundIdentity.java	74.02%	15 Missing and 5 partials ⚠️
...ava/tech/ydb/auth/OAuth2TokenExchangeProvider.java	94.07%	6 Missing and 2 partials ⚠️
...ain/java/tech/ydb/core/auth/StaticCredentials.java	84.00%	4 Missing and 4 partials ⚠️
...src/main/java/tech/ydb/auth/OAuth2TokenSource.java	94.80%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #282      +/-   ##
============================================
+ Coverage     44.39%   45.71%   +1.31%     
- Complexity     1571     1620      +49     
============================================
  Files           297      300       +3     
  Lines         11505    11807     +302     
  Branches       1124     1146      +22     
============================================
+ Hits           5108     5397     +289     
- Misses         5988     5999      +11     
- Partials        409      411       +2     

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[alex268]

Added examples of usage with a static token and with a RSA private key

[UgnineSirdis]

tokenEndpoint