!hco Make `Content-Length` and `Content-Type` visible but not constructible

This makes the `Content-Length` and `Content-Type` types visible - but not
constructible - outside of Akka HTTP. The change partially reverts #15800.
the http package. The types need to be visible so that Play can access
all parsed headers. However they types are not constructible outside Akka
HTTP because it is not desirable for users to create these headers. Users
should set the Content-Length and Content-Type via the members of the
HttpEntity objects.

Includes integration tests in the io.akka package to ensure that objects
have correct visibility outside akka.http.
Merge pull request #15895 from richdougherty/richd-http-integration-2.3

+hco Make `Content-Length` header public again and add integration tests
!hco make Marshaller(s) covariant in result type
!hco #15915 introduce more shades of HttpEntities

The introduction of BodyParts again showed that not all entity types are
useful for every kind of context. There are now these contexts where
HttpEntities are used:
 - requests
 - responses
 - body parts

And several kinds of entities:
 - Strict
 - Default
 - Chunked
 - CloseDelimited
 - IndefiniteLength

To increase type safety of the API marker-interfaces are introduced defining
which kinds of entities can be used in which contexts:
 - RequestEntity: Strict, Default, Chunked
 - ResponseEntity: Strict, Default, Chunked, CloseDelimited
 - BodyPartEntity: Strict, Default, IndefiniteLength

Also, to be able still to provide abstractions over some kinds of entities
additional auxiliary interfaces were necessary:
 - MessageEntity = RequestEntity >: ResponseEntity: Strict, Default, Chunked (type alias for RequestEntity)
 - UniversalEntity = RequestEntity with ResponseEntity with BodyPartEntity = Strict, Default
Merge pull request #15941 from spray/w/15915-restructure-HttpEntity-hierarchy

!hco #15915 introduce more shades of HttpEntities
!hco #15716: porting http internals to dsl2
Removed non-model usages of the old DSL