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 :white_check_mark:

Project coverage is 74.78%. Comparing base (c73b7f7) to head (b0171fa).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2994      +/-   ##
============================================
+ Coverage     72.93%   74.78%   +1.84%     
  Complexity     2781     2781              
============================================
  Files           139      112      -27     
  Lines         15166    11017    -4149     
  Branches       1022        0    -1022     
============================================
- Hits          11062     8239    -2823     
+ Misses         3552     2778     -774     
+ Partials        552        0     -552     

Flag	Coverage Δ
appsec-extension	?
tracer-php	74.78% <ø> (-0.02%)	:arrow_down:

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see 28 files with indirect coverage changes

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[morrisonlevi]

This was eventually moved into libdatadog.