HSv3 descriptor creation support

Cryptographically valid support for hidden service creation...

  https://trac.torproject.org/projects/tor/ticket/31823

HSv3 descriptors consist of three parts: an inner layer, outer layer, and the
descriptor itself. Callers of HiddenServiceDescriptorV3's create() and
content() methods can supply these to specify that layer's parameters.

For example, to supply custom introduction points with random key material
simply call...

  HiddenServiceDescriptorV3.content(
    inner_layer = InnerLayer.create(
      introduction_points = [
        IntroductionPointV3.create('1.1.1.1', 9001),
        IntroductionPointV3.create('2.2.2.2', 9001),
        IntroductionPointV3.create('3.3.3.3', 9001),
      ],
    ),
  )