Fixes for the C++ exports

[darbyjohnston]

This PR fixes a couple of issues with the C++ exports:

• Add missing OPENTIMELINEIO_IMPORT_TYPE macro
• Use the variable OTIO_SHARED_LIBS instead of BUILD_SHARED in src/opentime/CMakeLists.txt and src/opentimelineio/CMakeLists.txt
• Add a comment that you need to define OPENTIME_STATIC and OTIO_STATIC when building as a static library. These are defined automatically if you use the OTIO CMake config: find_package(OpenTimelineIO)
• Fix the typo: OPENTIMELINIO -> OPENTIMELINEIO

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.10%. Comparing base (c0e97b0) to head (c34086a).
⚠️ Report is 79 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    #1959      +/-   ##
==========================================
+ Coverage   84.11%   85.10%   +0.98%     
==========================================
  Files         198      181      -17     
  Lines       22241    12768    -9473     
  Branches     4687     1206    -3481     
==========================================
- Hits        18709    10866    -7843     
+ Misses       2610     1719     -891     
+ Partials      922      183     -739     

Flag	Coverage Δ
py-unittests	85.10% <ø> (+0.98%)	⬆️

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see 131 files with indirect coverage changes

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