Completion Service file spec interface should be more robust

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Currently, CompletionService.submitTaskWithFiles() allows for a file dictionary of the format:

{
    "my_key": "my_file_reference"
}

This is fine for simple default behavior without access control, but it would be much more robust if the value for any key could also support a dictionary. Perhaps a straw-man like this:

{
    "my_key": {
        # Allow the file reference from before, but this time as a key within the sub-dict
        "file_reference": "my_file_reference",

        # Allow a username for accessing the specific file reference.
        # This would allow for multiple files from multiple S3 accounts (say)
        # to be accessed by a single Completion Service Worker.
        "access_user": "my_access_user_name",

        # Allow password for accessing the specific file reference (see above)
        "access_password": "my_access_password",

        # Specify whether the worker requires this file to be present before execution.
        # This is intended to expose the equivalent of the 'immutable' flag to
        # CompletionService clients
        "present_before_execution": True
    }
}

It would be important to also support the original simple key/string-value for backwards compatibility.

[d1donlydfink]

potentially public