Add digester to enrich git annex metadata for every key

[yarikoptic]

Quite often we need to crawl and drop data right away due to prohibitive size. So we should take that short lived presence of the file as an opportunity to extract useful metadata, and knowing alternative checksums for a file could be of great use and also was envisioned as part of our "ultimate db". So I think it should be one of the options for any "annexification", so not every pipeline had to add the node. Digester already computes a good number of checksums

[yarikoptic]

I remembered now that git annex would copy metadata fields (in this case checksums) to a modified annexed file... Need to be able to annotate them to be key specific, and not copied . Todo: point to git annex issue