feat: add datadog-thin-str crate

[morrisonlevi]

It doesn't yet update profiling to use it. Splitting into multiple PRs for it to be easier to review.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.56%. Comparing base (978658e) to head (53a8da7).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2994   +/-   ##
=========================================
  Coverage     74.56%   74.56%           
  Complexity     2750     2750           
=========================================
  Files           111      111           
  Lines         10900    10900           
=========================================
  Hits           8128     8128           
  Misses         2772     2772           

Flag	Coverage Δ
tracer-php	74.56% <ø> (ø)

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