Multiple AddressableImportSettings support

[ese9]

Update with AddressableImportSettings. Make settings more flexible and readable if you have a lot of rules by spliting to different scriptables.

• add multiple AddressableImportSettings support
• add rules enable state toggle
• add ability to place settings asset in any project folder (not Assets/AddressableAssetsData only)

Closes #66

[favoyang]

Hi @ese9,

Thanks for the contribution. I like the idea. But here's something to be considered:

1. Instance(s) is being invoked every time an asset is imported. I suppose AssetDatabase.FindAssets("t:... will be a bottleneck for a larger project (especially when performing a bulk reimport).
2. The ordering of multiple settings should be deterministic. i.e. by introducing a priority field or referencing and ordering multiple settings files into a singleton lister file.

[favoyang]

Please also notice that https://github.com/favoyang/unity-addressable-importer/pull/68 brings some conflicts for the PR.