[WIP] Composite EC2 Plugin for Metadata

[naftulikay]

Tags also will be forthcoming, but in a different PR.

Strategy

• Use a HEAD request to http://169.254.169.254/ and examine response code and Server header, expect EC2ws, otherwise stop detecting facts.
• Instead of using a recursive stack approach, use a generator and return a list of (String, String) where the first value is the path and the second value is the payload.
• For /public-keys/, each entry will be %d=%s, where %d is a zero-indexed number and %s is the name of the key. When recursing, check public-keys/%d/%s to get the key fingerprint.
• For JSON responses, attempt to deserialize into a Map.

[naftulikay]

trying to reason about the futures lifecycle: