Support disable standard contexts at event-level

[colmsnowplow]

What I have in mind is an argument in the event methods that disable standard contexts being sent with that event.

It would be useful to only send standard contexts with their relevant events, from a volume management point of view.

Additionally, in an implementation where there's a disconnect between the data analytics/BI team and the implementation team, the data would be simpler to consume if contexts were disabled where they aren't required.

(In other words, it would be useful to set up your tracking such that each event is sent with only those contexts which relate to it - especially where you've got a bigger team/distance between implementing tracking and consuming data)