[#627 expired SAS tokens] Runner downloads task JSON from URL

[BMurri]

Problem:
The runner today can both upload AND download files from blob storage by obtaining the needed SAS at the point of need, but it cannot obtain the task JSON that way, which blocks the way to no longer use any SAS tokens in batch task configurations, due to the fact that it requires the task JSON in the local filesystem.

Solution:

1. Enable obtaining the authentication metadata (for both TERRA and CoA/standalone deployments) via environment variables.
2. Allow the task JSON to be provided via the command-line as a URL.
3. When the task JSON is a URI, use the current mechanism for downloading files to download the URL and use the content as the task JSON.
4. Consider whether the task JSON should be placed in/near the working directory (in the local filesystem) or if it should be streamed directly to memory and held there until final task completion.

AC

• Unit tests to verify correct behaviors
• Consider integration tests to demonstrate correct behavior.