outbound-http: Support "relative" URIs to current app

[lann]

This has been requested a couple of times and should be relatively straightforward:

• Add a field to OutboundHttp e.g. origin: Uri which gets updated from each inbound request
• If an outbound request starts with a / (has no scheme or authority), join with the origin

[itowlson]

If the application has a base, should "relative" URLs be relativised to that base, or to the host? (The reference to "current app" makes me assume this is relative to the app base but I want to check!)

[lann]

First, I'd like to get rid of base 😅.

I'm not sure which would be less confusing. Magically prepending things to /path seems like it might be more surprising, even if it is logical from "inside the app".

[radu-matei]

ref #957