feat(core): add context.http and move statusCode there

[vonagam]

This is context.http from #2484.

Here it contains only statusCode (can be renamed to status or not, don't have an opinion one way or another so kept previous name), with the idea to add responseHeaders later (redirect is not needed since it will be covered by those two).

I used getter/setter to prevent having two sources of truth and things getting out of sync. Also ideally a depreciation message should be added to those accessors that will write a warning first time one of them is accessed (a helper function for such depreciations can be added to @feathersjs/commons).

[daffl]

This looks good, thank you! Just two small somments.