add optional `outputFilename` property to crawlpath interface

[TimDaub]

• see these two PRs:
  • 3448da95b3d81396caf2ec558759da5cd4134dc4
  • 83959659f00251ac13a0418b5877fbffd11e27d3
• we should check if it's a string
• potentially we should check that it DOESN'T include "extraction" or "transformation" as these terms will be post-pended in lifecycle.