Enhance task export_csv: address Drupal config where media file url is not included in JSON serialization of media

[jefferya]

Link to Github issue or other discussion

#1025

What does this PR do?

Enhances the export_csv task by:

1. Add alternate method to find media file url via the entity/file; address export_csv task not finding media #1025. Some Islandora sites don't respond with a media file url property in the field_media_document, field_media_video_file, field_media_audio_file, field_media_file, etc., sections of the JSON response during a request to node/{id}/media?_format=json. Not reproducible on the Islandora Starter site.

2. Reduces noise in the workbench log by only testing if the export directory exists during initialization instead of each time a media is downloaded thus reducing workbench.log messages indicating "Path "export_file_directory" ("export_files") already exists."

What changes were made?

Enhances the export_csv task by:

1. If the url property is not found via node/{id}/media?_format=json API then checks the entity/file API.
2. Reduces noise in the workbench log: creates an export validation method that runs during initialization replacing validation that run during each media download.

How to test / verify this PR?

1. media file url
   a. should work against the islandora.sandbox.ca where media file url is part of the node/{id}/media?_format=json API
   b. should download files on the Drupal site configured such that the media file url is only exposed by the entity/file API

2. export_csv task: workbench log output should only have one message for the following cases (not one for each media file downloaded)
   a. test config without export_file_directory
   b. test config with export_file_directory pointing to new directory
   c. test config with export_file_directory pointing to an existing directory

Sample config

task: export_csv
host: "https://cwrc.ca"
username: admin
 
content_type: islandora_object
 
input_dir: /tmp/test1/
input_csv: input.csv
 
export_csv_term_mode: name
 

export_csv_file_path: /tmp/test1/islandora_object.output.csv
 
export_file_directory: /tmp/test1/islandora_object