Support writeKey in event body

[kevburnsjr]

Segment's documentation recommends setting an Authorization header

https://github.com/rudderlabs/rudder-server/blob/b834143704838cfff7dd01d19b13cbab3b8af2f4/gateway/gateway.go#L147

However, there is at least one other way to supply a write key that is supported by Segment's API.

1. Supplying a write key in the writeKey property of the event body will register as a valid event of the specified type. The writeKey in the event body will override the write key in the Authorization header if present.

2. Also... Each item in a Batch may contain a different write key from its parent. iirc, Segment will use the batch write key by default and overrides with event specific write key for each event if present.

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This API has been designed for maximum interoperability; best to make all possible accommodations.
We never know who might have rolled their own client relying on some quirk like this (they exist).