4.x: Refactor Http class

[tomas-langer]

Description

Move inner classes from io.helidon.http.Http to stand alone classes.
This is to prepare for HTTP declarative endpoints, where annotations will be inner classes of Http class.

Each commit is one class moved. Last commit is tying up loose ends

Documentation

I did not refactor documentation, there are a few places that use these types and must be fixed.

The following types have been moved (from -> to):

1. Http.Method -> io.helidon.http.Method
2. Http.Status -> io.helidon.http.Status
3. Http.HeaderName -> io.helidon.http.HeaderName
4. Http.HeaderNames -> io.helidon.http.HeaderNames
5. Http.Header -> io.helidon.http.Header
6. Http.HeaderValueWriteable -> io.helidon.http.HeaderWriteable
7. Http.Headers -> io.helidon.http.HeaderValues
8. Http.DateTime -> io.helidon.http.DateTime