Add standard test EdgeRequestContexts

[pacejackson]

One problem we almost always run into when writing tests for baseplate services is having to create sever edge request context headers to test our endpoints. It would be nice if baseplate could provide a standard collection of these values that could be used by each service.

[fishy]

I guess since we moved edgecontext to its own repo, we no longer need this one? (maybe we can duplicate it there 😅)