Allow build system to use rapidjson from the system

[jlskuz]

Summarize your change.

For (Linux) distro packaging it is common to use system packages instead of submodules and co.

With this option one can ask to use the system rapidjson with CMake flag instead of maintaining custom patches such as https://build.opensuse.org/projects/KDE:Applications/packages/opentimelineio/files/0001-Use-system-rapidjson.patch?expand=1

Note: if OTIO_FIND_RAPIDJSON is ON but rapidjson is not found cmake fails. This behavior is different to OTIO_FIND_IMATH where it will just continue with src/deps as a fallback. However I think if the user explicitly request the system package it makes more sense to fail if not found. In my opinion this should be changed for Imath as well, but that is offtopic for this PR

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.72%. Comparing base (c0e97b0) to head (b8590aa).
⚠️ Report is 92 commits behind head on main.

❌ Your changes status has failed because you have indirect coverage changes. Learn more about Unexpected Coverage Changes and reasons for indirect coverage changes.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1859      +/-   ##
==========================================
+ Coverage   84.11%   84.72%   +0.60%     
==========================================
  Files         198      177      -21     
  Lines       22241    12807    -9434     
  Branches     4687     1191    -3496     
==========================================
- Hits        18709    10851    -7858     
+ Misses       2610     1773     -837     
+ Partials      922      183     -739     

Flag	Coverage Δ
py-unittests	84.72% <ø> (+0.60%)	⬆️

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see 131 files with indirect coverage changes

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[jlskuz]

The readthedocs.org failure seems unrelated?

[darbyjohnston]

Unrelated; sometimes readthedocs has these failures, possibly a network error. I'm not sure how to re-run the job, it's separate from the Github actions. Maybe wait for a bit and try pushing another change to try again.

[ssteinbach]

I restarted the RTD build - sometimes that does flake out, sorry

[darbyjohnston]

Thanks for the changes; I agree about the fallback, the Imath behavior could be changed in a separate PR.