Add support for `httpPayload` in server responses (#1158)

Note that with this change, payload serializers now return `Vec<u8>` or
`ByteStream` instead of always `SdkBody`, since the server does not use
`SdkBody`. The caller then needs to convert the byte slab into the
appropriate HTTP body type. To this end, `HttpBoundProtocolBodyGenerator`
has been extracted into its own file and renamed to
`HttpBoundProtocolPayloadGenerator`.

This commit also makes the body types exports from
`aws-smithy-http-server` `#[doc(hidden)]`, since they should only be
used by generated code, and exports all of them from `body.rs` instead
of `lib.rs`.

Comments have been added where appropriate to document which serializers
are called by the client / server and how their use differs.