Add file checksum controller

[mjordan]

Necessary for mjordan/islandora_workbench#34.

[mjordan]

This is the JSON returned when a file is created by create_file():

{'fid': [{'value': 1574}], 'uuid': [{'value': '3a9500ee-6269-42e3-bd08-9aabf2bacfcb'}], 'langcode': [{'value': 'en'}], 'uid': [{'target_id': 1, 'target_type': 'user', 'target_uuid': '7459b795-c0ee-4975-ae01-a2fb4658d970', 'url': '/user/1'}], 'filename': [{'value': 'IMG_5083.JPG'}], 'uri': [{'value': 'public://2021-07/IMG_5083.JPG', 'url': '/sites/default/files/2021-07/IMG_5083.JPG'}], 'filemime': [{'value': 'image/jpeg'}], 'filesize': [{'value': 1910313}], 'status': [{'value': False}], 'created': [{'value': '2021-07-21T05:35:43+00:00', 'format': 'Y-m-d\\TH:i:sP'}], 'changed': [{'value': '2021-07-21T05:35:43+00:00', 'format': 'Y-m-d\\TH:i:sP'}]}

The uuid in the above JSON is for the file.

Within create_islandora_media(), we need to GET the media using its mid, and the returned JSON contains the file's UUID.

Note: confirm that files stored in Fedora return the checksum (they appear to).