Improve transformer service

Dev Story :
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New transformers :
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There are pleinty of command line tools that can be used to handle transformations.
I would to have a easy and possibily cross platform way to integrate these third party tools as Nuxeo transformers.

Tranform blobs and Documents :
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API should expose an API to handle transformation from a DocumentModel.
Event is reading blob from a DocumentModel is not the only use case, this is a main use case.

Cache management :
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I don't want to handle cache "by hand" at application level. Transformation service should deal with that.

Implementation Notes :
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New transformers plugins :
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Some addons inlcude dedicated transformers :

• Any2Html for HTML preview
• Picture transformers for PictureBook and Tiling
• Base class for CommandLine based transformers
• Zip2Text

=> generic transformers should be part of global plugins

• this will improve the default transformers set
• this will make configuration more centralized

Cache management :
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New Addons working with transformers have to handle caching.
This caching is handled at application level.

=> This would be far mor efficient to have a cache management at transformer level
==> This requires some improvement of Transformer API

• input API should contain needed information to generate a cache key
• input API should contain a way to ask for cache invalidation (ie : fresh transform)
• output API should include API about cache status

API :
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Titling include a ImageResource API for wrapping a Blob or a Blob in a DocumentModel.
This includes cache key management.
We should leverage this to make a TransformerResource.