export header

[rixmann]

An export must be pre-/post-fixed with a header/footer.
preferably it should be a header but at the beginning of an export not all data may be available (at least not in an efficient and consistent way).

the following attributes should be included:

• schema version
• schema
• timestamp
• used filter (where: )
• exported record count
• username triggering the export
• machine: host/maschine_id, erlang nodename, release version
• jobid
• environment: database name and version

some attributes may require further discussion or may be dropped because they are not available in the context of ecto_export but must be handled by exd.