Make save mode configurable per allowed submitter for Bulk Submit

[johngrimes]

The Bulk Submit import currently hardcodes the save mode to merge when importing submitted data. This should be configurable at the allowed submitter level, so that different submitters can use different save modes (e.g. overwrite, append, merge) depending on the nature of their data.

The available save modes are: error, overwrite, append, ignore, and merge.

The configuration would be added to each entry in pathling.bulkSubmit.allowedSubmitters, for example:

pathling:
  bulkSubmit:
    allowedSubmitters:
      - system: "https://example.com/systems"
        value: "hospital-ehr"
        saveMode: overwrite
      - system: "https://example.com/systems"
        value: "clinic-system"
        saveMode: merge

The default should remain merge for backwards compatibility.