Add support for plugin version numbers

[apetrynet]

As part of the discussion around issue #1386 it has come up that it would be good to be able to check the version of a loaded plugin.
This PR adds a version field to the PythonPlugin class. The version number is defined in the manifest.

otiopluginfo displays the version number next to the adapter name. For plugins that don't contain the version key "unknown" is listed.

To my knowledge, this PR does not break backwards compatibility so I left the PythonPlugin schema version number as is. Please comment with your thoughts on this.

Example version definition in manifest:

{
            "OTIO_SCHEMA" : "Adapter.1",
            "name" : "svg",
            "version": "1.0.0",
            "filepath" : "svg.py",
            "suffixes" : ["svg"]
        }

Example output from otiopluginfo:

[...]
adapters:
  cmx_3600 - version: unknown
    doc (short): OpenTimelineIO CMX 3600 EDL Adapter
    path: /home/daniel/Code/OpenTimelineIO/.venv/lib/python3.9/site-packages/opentimelineio/adapters/cmx_3600.py
    from manifest: /home/daniel/Code/OpenTimelineIO/.venv/lib/python3.9/site-packages/opentimelineio/adapters/builtin_adapters.plugin_manifest.json
    explicit supported features:
      read_from_string args: ['input_str', 'rate', 'ignore_timecode_mismatch']
      write_to_string args: ['input_otio', 'rate', 'style', 'reelname_len']
    implicit supported features:
      read_from_file (calls: read_from_string)

  svg - version: 1.0.0
    doc (short): OTIO to SVG Adapter
    path: /home/daniel/Code/OpenTimelineIO/.venv/lib/python3.9/site-packages/opentimelineio/adapters/svg.py
    from manifest: /home/daniel/Code/OpenTimelineIO/.venv/lib/python3.9/site-packages/opentimelineio/adapters/builtin_adapters.plugin_manifest.json
    explicit supported features:
      write_to_string args: ['input_otio', 'width', 'height']