feat: change how the response handle streams

[francescovallone]

The Response.stream in the latest version of Serinus is not behaving as a Stream but more like a facade on top of the Response.text. This must be changed in the view of a more organic approach to how things are handled in the whole framework.

For this reason the Response.stream must undergo to a refactor with all the changes that it requires to work.
The idea is:

• The Response.stream closes the request only when the stream associated with it has ended or when the handler has returned;
• The Response.stream accepts a stream of whatever data the user wants but it will all be converted to Uint8List when sent.

[francescovallone]

First try