Added opt-in features

[glen-84]

Summary of the changes (Less than 80 chars)

• Added opt-in features.

Closes #7720

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❓ Questions:

• Please check the schema error definitions in ErrorHelper.cs.
  • Should I be setting an error code or specified by?
• I'm returning one error for each application of the directive, is that correct?
  • In theory, we could be including information about the directive syntax position, arguments, etc.

📓 Spec notes:

• Should we explicitly disallow empty and whitespace-only feature strings? *
• Should we disallow whitespace or other characters within feature names, or leave it completely open? *
• I've used includeOptIn instead of includeRequiresOptIn for introspection, as it seemed more clear, WDYT?

* stability strings as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.13%. Comparing base (bf43444) to head (2f31b5a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7652      +/-   ##
==========================================
+ Coverage   74.05%   74.13%   +0.07%     
==========================================
  Files        2920     2931      +11     
  Lines      151493   151920     +427     
  Branches    17614    17661      +47     
==========================================
+ Hits       112194   112622     +428     
+ Misses      33445    33435      -10     
- Partials     5854     5863       +9     

Flag	Coverage Δ
unittests	74.13% <ø> (+0.07%)	⬆️

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