Add in metadata for dataset submissions

[ialarmedalien]

The schema should be expanded to allow the addition of dataset metadata. At minimum, this should include:

• timestamp for dataset production
• version of the BERtron schema that it is compatible with
• data source URL
• version of that data source (if available/applicable)
• contact info for the dataset (email address)

Suggested structure:

meta:
  timestamp: 2025-09-17T18:29:01Z
  berton_schema_version: 0.12.0
  data_source: https://data-source.com
  version: 1.2.3   # optional
  contact: curators@data-source.com

data:
  # data goes here

Alternatively the metadata could be supplied in a separate file but it's probably best to keep everything together.