CBST2-12: Improve External HTTP Requests

[jclapis]

Do not merge until #310 is complete, as this relies on it.

This is a fairly large PR that addresses numerous issues with the way external HTTP requests are handled:

• Enforces that https is used as the scheme of any endpoints to prevent man-in-the-middle listeners
• Adds a configurable timeout so requests don't run indefinitely
• Reads response bodies in chunks instead of loading them all directly into memory before using them
  • Prevents handling if the Content-Length header is too large
  • If it's missing, stops if reading the response body has exceeded a size threshold (currently 10 MB)

This also modifies the SSV loader tests to use a local server with a captured (good) response instead of the canoncial SSV server.