Add context field to HTTPServerRequest

[thaven]

The context field allows application specific data to be passed down the chain of request processors.
Solves #1529

[wilzbach]

@thaven your PR needs a rebase.

@s-ludwig what's your opinion on this? On #1529 there are many people in favor of a way to store custom information within the current request.

[s-ludwig]

Definitely makes sense, I even remember having implemented this once, but for some reason it then got deferred. I'm just trying to remember now which name I've used and whether "context" is any better or worse.

One thing regarding the implementation: DictionaryList!(Variant, true, 4) would probably be a better idea than Variant[string], so that a few entries can be set without additional GC allocations.

[schveiguy]

One thing regarding the implementation

Sounds good, I really look forward to this update! @s-ludwig, you can just edit the PR branch, no? It's possible @thaven isn't working on this any more as the PR is so old.

[thaven]

I do not have much spare time at the moment, so feel free to edit the PR.
@s-ludwig: Using DictionaryList seems a good idea to me.

[s-ludwig]

@thaven: Alright, I'll change to DictionaryList and merge!