[API Review]: Move BadHttpRequestException to Http.Abstractions

[davidfowl]

Kestrel and IIS both expose public BadHttpRequestException types, if there's no good reason for those 2 types to be different, lets unify and push it further down the stack.

cc @halter73 @jkotalik

[halter73]

I was going to suggest that IIS just use Kestrel.Core version for max compat, but I'd be shocked if anything on our tests actually depend on this. I think we should just "break" in 3.0 and pick a shared namespace.

[jkotalik]

IMO this is a nice to have, but I doubt we will have time 😄

[halter73]

I feel most of the time spent on this will be deciding whether or not to do it.

[davidfowl]

Agree with Stephen, we should do it now.

[analogrelay]

Doesn't feel valuable for 3.0 and we don't have time for it in preview7