Make ui and rest proper packages

[ZanSara]

As noticed with #2086, the dependency management of the UI and REST_API was suboptimal, as it treated the dependencies of these two packages as dependencies of Haystack itself. However, the UI does not even depend on Haystack, while the REST API should install Haystack as their own dependency, rather than the contrary.

As a solution I propose:

• Make a setup.py file for the ui/ folder.

  • From now on, the UI will be installed (after a git clone) with pip install ui/ (or pip install . from the ui/ folder).
  • Installing the UI will not install Haystack.
  • There is no more pip install farm-haystack[ui].
  • In this case I chose a setup.py file because apparently a package cannot be installed if it lacks both setup.py and pyproject.toml. Given that both would be basically empty, but setup.py is still required for editable installs, I chose to keep only setup.py.

• Make a setup.py for the rest_api/ folder

  • From now on, the REST API will be installed (after a git clone) with pip install rest_api/ (or pip install . from the rest_api/ folder).
  • There is no more pip install farm-haystack[rest].
  • Installing the REST API will install the local Haystack that was git cloned with the rest_api folder.
  • The above feature required me to use setup.py instead of setup.cfg, because it needed to look up into the parent directory for farm-haystack (something that seemingly setup.cfg can't do).

On top of the above changes, this PR also updates the Dockerfiles to make use of this new setup. Version information is also aligned with the version of Haystack itself.

Note that for the time being, I left both packages without license information, as they were lacking it before as well. However this might become a trouble for companies. @tholor shall I add an explicit Apache 2.0 to these two packages as well?

Closes #2086

[julian-risch]

LGTM! I agree that we should include license information in rest_api and in ui.