Added environment auth provider with runtime class loading

[alex268]

Fixes #308

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 36 lines in your changes missing coverage. Please review.

Project coverage is 47.26%. Comparing base (ebe9f19) to head (e6bc656).

Files with missing lines	Patch %	Lines
...n/java/tech/ydb/core/auth/EnvironAuthProvider.java	0.00%	32 Missing ⚠️
.../src/main/java/tech/ydb/auth/OAuth2AuthHelper.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #339      +/-   ##
============================================
- Coverage     47.40%   47.26%   -0.14%     
  Complexity     1750     1750              
============================================
  Files           309      311       +2     
  Lines         12448    12484      +36     
  Branches       1232     1237       +5     
============================================
  Hits           5901     5901              
- Misses         6087     6123      +36     
  Partials        460      460              

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