Allow adding custom environment values on DHSProto

[Ozaq]

Description

DHSProtocol now allows passing in custom environment values to be sent to the server.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.20%. Comparing base (736ce0d) to head (7df53bc).

Files with missing lines	Patch %	Lines
src/metkit/mars/DHSProtocol.cc	0.00%	10 Missing ⚠️
src/metkit/mars/RequestEnvironment.cc	0.00%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #109      +/-   ##
===========================================
- Coverage    68.32%   68.20%   -0.13%     
===========================================
  Files          124      124              
  Lines         7340     7353      +13     
  Branches       732      732              
===========================================
  Hits          5015     5015              
- Misses        2325     2338      +13     

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