Send initiator identity and other configured metadata with upstream requests

[dannykopping]

Following on from #126, we need to send the initiator of the request to the upstream, so customers can stand a proxy up in between Bridge and whatever AI provider they are using to track or conditionalize access.

The header should take the form X-AI-Bridge-Initiator-Id=....

Additionally, any metadata provided to the Actor (context/context.go) should be included in the form X-AI-Bridge-{Key}={Value}.

These headers should only be injected if a flag is set to true.