Populate_Metadata: add support for bulkmap context

[sbesson]

See https://forum.image.sc/t/are-attributes-in-an-images-table-searchable/27999/5

In addition to the table generation functionality, a future extension of the Populate_Metadata script (or a separate script if needed) would be to be convert the table into key/value pairs via a script.

Immediately, this could achieved either by attaching a file containing the configuration file to use for the conversion. Depending on ome/omero-metadata#33, the script could also use a default configuration to turn every column of every row into a key/value pair.