Rework HTTP Client Configuration support

[philwebb]

We have a number of related issues that we can't easily fix until we redesign the way that HTTP Clients are configured. See theme: http-client-config.

[anmol797]

Hi , can i also contribute here

[wilkinsona]

Thanks for the offer, @anmol797, but as indicated by the status: pending-design-work label, we need to do some design work here before we can consider accepting contributions. It's also already assigned.