Add Mode field to the ConformanceReport API

[mlavacca]

What would you like to be added:

#2391 is a GEP update that proposes to introduce a new mode field in the report to give implementations the possibility to run the conformance suite in environments using different settings, and properly specify the settings under the conformance tests run.

Such a new mode field has to be added to the API, and the conformance suite should be improved to have a way of customizing such a field.

/area conformance

[mlavacca]

/assign