⚗️ cdviz-collector: add transformers ( eg file/event info, into cdevents) for sources

[David Bernard]

To allow more customizable sources and reuse the polling, ... try extract the transformer and to provide several for some sources.

To experiment start with opendal source an transformer to convert file into cdevent:

• read content as cdevent json (current behavior)
• file metadata converted into a cdevent via a template (handlebars?)

Notes:

cdviz-collector is a ETL pipeline, so having clear transformer definition looks like a good idea