HeaderPropagation: add support for hosted services

[alefranz]

This is a possible solution to fix #12169, adding the ability to use a HttpClient with the HeaderPropagationMessageHandler outside of a http requests.
It also adds the ability to reuse the functionality of the HeaderPropagationMiddleware in different scenarios, for example when consuming messages from a queue, however I guess this scenario is out of scope for a middleware.

• Extract the functionality from the Middleware to a generic Processor that can be called outside a http request.
• changed the valueFactory to receive a headers collection instead of the HttpContext

The main issue I see with this approach is that when the processor is used directly you must take extra care as you need a different async context per request.

Addresses #12169

/cc @rynowak

[analogrelay]

This is a pretty old draft. Where do we stand here? @alefranz @rynowak ?

[alefranz]

Hi Andrew,

This has been partially addressed with #12636, as now you get a proper exception when trying to consume this from a hosted service.
From the conversation in #12169 there was no interest to extend the functionality to support the Hosted Services scenario. If this is still the case, we can close both this PR and the related issue.

Thank you.

[analogrelay]

@alefranz I don't think we necessarily need to close the issue, but I think I will close this PR since it's gotten pretty stale and would need significant rebasing anyway. If we come to an agreement on the issue that we do want this functionality and how we want to implement it, I don't see any problem with opening a new PR with this change.