Add route describing different export formats

[anne17]

Add another route that provides information about the different Sparv export formats. This can be used by the frontend. Right now it's hard for a user to know what's what.

[anne17]

Just realized that we already have something like that in the backend: https://ws.spraakbanken.gu.se/ws/mink/sparv-exports. This information is computed by sending the sparv run -l command on the command line and parsing the output.

From the /list-exports call we get file paths like csv_export/dokument2.csv and xml_export.pretty, but it is not obvious how they should be mapped to the information from the /sparv-exports call where we get things like "export": "csv_export:csv" and "export": "xml_export:pretty".

Any ideas @MartinHammarstedt?

[MartinHammarstedt]

We could have a verbose version of the export listing in Sparv (maybe in JSON format for easier parsing), which could include output info about the exporters. It would look like this for exporters with one export file per source file: export/xml_export.scrambled/{file}_export.xml.

[MartinHammarstedt]

sparv modules --exporters --json now gives this info in a machine readable format.