feat: add future flag to experimental_features

[cofin]

Description

This adds a future flag to experimental_features to enable new features that may be otherwise considered breaking.

Closes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.41%. Comparing base (06024fe) to head (0469724).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3864   +/-   ##
=======================================
  Coverage   98.41%   98.41%           
=======================================
  Files         344      344           
  Lines       15475    15478    +3     
  Branches     1702     1702           
=======================================
+ Hits        15230    15233    +3     
  Misses        115      115           
  Partials      130      130           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/3864