[KATANA] Asset API

Concept:

                    asset plugin

Katana <---------------------->  Asset Management System 

                 retrieve, publish
             browser & parameter widget customization

Workflow:

           shot#, seq#, ...            ------>   Asset ID 1  ----->   File Path

asset fields (node parameters)      buildAssetId       resolveAsset

if for retrieving asset, just load the asset/scene

if for publishing asset,
------> Generate Asset   ------>  Asset ID 2 (with increased version or override current one)

    createAssetAndPath                                postCreateAsset(... assetType, fields, args)