Enable Libheif Plugin System

[bigcat88]

Ref: #154

Added the load_plugin function for loading single plugins (most likely libde265 will move into a separate plugin in the future if possible)

Option DENABLE_PLUGIN_LOADING is now ON so now libheif will automatically try to load plugins from "LIBHEIF_PLUGIN_PATH" environment variable or from the build-in path(but it will be different in each system, so not much use of it), imho.

This does not bring any other special changes, this is more work for trying to take x265, aom out into a separate projects to make pillow_heif more configurable and easy to develop.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a45f845) 100.00% compared to head (b316649) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #205   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines          923       925    +2     
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+ Hits           923       925    +2     

Files	Coverage Δ
pillow_heif/__init__.py	100.00% <100.00%> (ø)
pillow_heif/misc.py	100.00% <100.00%> (ø)