Add the ability to insert header from the context in the header layer

[bnjjj]

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Following a discussion and suggestion from @Geal it could be great to add the ability for a user to insert a header from the context in the header layer

[abernix]

@bnjjj Do you mind taking another look at this when you have a chance? Is this still relevant or something that we should talk about? (Feel free to close if you think it's settled!)

[bnjjj]

I think yes it could be still relevant ! The idea is to be able to add a header with a value fetched from our extensions given a key. It's not a big priority and could be deferred

[abernix]

To quote a bit from a non-GitHub convo:

In general, would be interested in finding a way for plugins to communicate with each other. E.g., a notification plugin might need to get the identity and put that into the context for another plugin to see, "okay, do I need to assign something else to a subgraph? do I need to integrate that inside the query exection?". a way to put something into the context and get something from the context, too. -@Geal