Add support for extensible input enumerators

[bradlarsen]

This adds extensible enumerators to Nosey Parker. This mechanism makes it possible to scan inputs produced by any program that can emit JSON objects to stdout, without having to first write the inputs to the filesystem.

It is invoked with the new --enumerator=FILE option, where FILE is a JSON Lines file. Each line of the enumerator file should be a JSON object with one of the following forms:

  { "content_base64": "base64-encoded bytestring to scan", "provenance": <arbitrary object> }
  { "content": "utf8 string to scan", "provenance": <arbitrary object> }

Shell process substitution can make streaming invocation ergonomic, e.g., scan --enumerator=<(my-enumerator-program).