Use std::optional if available.

[asopov-here]

Currently, SDK requires C++11 minimum.
So, boost::optional type is used for optional values. For C++17 and above more convenient is to use std::optional instead. The task NLAM-23 is about making this type configurable. This commit is a second part of the task: olp-cpp-sdk-authentication.

Relates-To: NLAM-23

[codecov]

Codecov Report

❌ Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.29%. Comparing base (a387a38) to head (fef9e8d).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...k-authentication/src/AuthenticationClientUtils.cpp	66.67%	1 Missing ⚠️
olp-cpp-sdk-authentication/src/TokenResult.cpp	66.67%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1625      +/-   ##
==========================================
- Coverage   80.30%   80.29%   -0.02%     
==========================================
  Files         349      349              
  Lines       13937    13940       +3     
  Branches     1491     1492       +1     
==========================================
  Hits        11192    11192              
- Misses       2146     2150       +4     
+ Partials      599      598       -1     

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