[backend][assetmodel] Made AssetType a python enum

This will make the enum itself more usable. Style wise, this makes
it an independent element of AssetModel.
2 jobs for asset_type_simplification in 14 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #159072
docker
build_linux_36

00:04:46

passed #159073
macosx
build_macosx_36

00:14:07