chore: add a mechanism to set multiple envs per configuration

[mabdinur]

What does this PR do?

• Updates OptionDefintion.env to accept a list of environment variables. With this change dd-trace-rb contributors can supply multiple envars for each configuration (in order of precedence).
• Introduces resolved_env instance method for the Options class. This will allow developers to detect which environment variable were used to set a configuration.

Motivation:

Additional Notes:

How to test the change?

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.10%. Comparing base (f7a8fb5) to head (7b60462).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3679      +/-   ##
==========================================
- Coverage   98.11%   98.10%   -0.01%     
==========================================
  Files        1225     1225              
  Lines       72519    72543      +24     
  Branches     3446     3448       +2     
==========================================
+ Hits        71149    71171      +22     
- Misses       1370     1372       +2     

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