RV adapter fix: don't require having access to OTIO from python interpreter in rv

[ssteinbach]

Originally OTIO was a dependency free python library. Over time, we've made OTIO into a C++ backed thing and can no longer guarantee that the library is compatible across the RV barrier. This shifts the RV adapter around so that a simplfied manifest json is handed across to a small script inside RV that can run the RV commands.

[codecov-io]

Codecov Report

Merging #941 (2299f52) into master (cd2b80b) will decrease coverage by 0.12%.
The diff coverage is 10.96%.

@@            Coverage Diff             @@
##           master     #941      +/-   ##
==========================================
- Coverage   85.77%   85.65%   -0.13%     
==========================================
  Files         191      191              
  Lines       18108    18153      +45     
  Branches     2048     2058      +10     
==========================================
+ Hits        15533    15549      +16     
- Misses       2053     2082      +29     
  Partials      522      522              

Flag	Coverage Δ
unittests	85.65% <10.96%> (-0.13%)	⬇️

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Impacted Files	Coverage Δ
...ntrib/opentimelineio_contrib/adapters/extern_rv.py	0.00% <0.00%> (ø)
contrib/opentimelineio_contrib/adapters/rv.py	15.97% <14.04%> (-10.96%)	⬇️

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