Add Transformer class for live HTTP objects

[mikkokar]

To make Styx APIs safer for the end-users, this PR adds a separate Transformer alongside Builder in LiveHttpRequest and LiveHttpResponse classes. The transformer restricts what the API consumers can do 1) when creating a new live HTTP object and 2) when transforming one.

Some examples:

It is no longer possible to replace, and thus disconnect the reactive ByteStream chain like so:

    response().build()
            .newBuilder()
            .body(new ByteStream(...)).   // No longer available
            .build();

The transformer class now enforces API consumers to transform the byte stream with the lambda alternative. Like so:

    response().build()
            .newBuilder()
            .body(ByteStream::drop)
            .build();

Likewise it is no longer possible to supply a mapper lambda when creating a live object. This is no longer possible:

    response()
            .body(stream -> doSomething(stream))  // No longer available
            .build();