Improve the stream importer to support image and video

The current implementation generates only random text blobs.

We want to create images and videos document.

The idea is to use a list of sample files (mp4 and jpg) as an input and be able to generates any number of valid images or videos files by adding some watermark.

A mass import is done in 4 steps with the stream importer:
1. produce BlobMessage in a blob stream.
2. consume the blob stream, import blobs into the binary store, produce BlobInfo into a blob-info stream
3. consume the blob-info stream and produce random DocumentMessage in a document stream
4. consume the document stream and create documents in Nuxeo

We need to improve the 2 first step by creating a

• FileBlobMessageProducer that use a file listing as input.
• BlobWatermarkMessageConsumer reading blob file and add some random watermark before importing the blob

The steps 3. and 4. does not need to be changed.

Watermark support should be done at least for jpeg and mp4 files adding some metadata that can be read is enough.

See https://github.com/nuxeo/nuxeo-platform-importer/tree/feature-SUPINT-915-importer for a first implementation.