Add detailed download reporting to AetherGrapeEngine

Groovy's Grape allows a user to enable download reports using the
system property groovy.grape.report.downloads. This commit updates
AetherGrapeEngine to honour this property and produce a detailed
download report when the system property is set to true. In the
absence of the system property, or when it's set to a value other than
true, the existing summary report is still produced.

[bs-344]
[60145094]