This was previously part of the `securesystemslib.keys` interface.
To prepare for deprecation of this interface, and use of the new
signer API as replacement, spx-related functionality and test are
moved.

This is a backwards incompatible change and a slight feature
degradation:

- spx is now no longer available via the keys module, and
thus no longer available SSlibSigner/SSlibKey (i.e. the legacy
API bridge). Given that spx was added recently, support via the legacy
API does not seem necessary. The advantage is more modular code and
less case-handling, also wrt different optional dependencies.

- Currently, SpxSigner does not implement from_priv_key_uri, which
was previously available via SSlibSigner, and supported loading
signers from file or envvar. Support may be added back later.

- The default keyid is computed differently than before (see
6c29cae)

See usage example in docstring.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

* Removes previously added SpxSigner.new_ key pair generation method.
  This method may be convenient to quickly test spx signing, but does
  not fit well into the signer API usage pattern, where key generation;
  public key and signer URI loading; and signer loading (from URI) plus
  signing are separate workflows.

* Adds function to generate and return raw key pair bytes.
  * This function is "syntactic sugar" (quote @jku) over the PySPX library.
  * It is added here to provide consistent error handling, if the
    optional library is not available.
  * Otherwise, it needs additional tooling to fit well into the signer
    API, e.g. an interface that stores the created private bytes to a
    place from where they can be loaded using Signer.from_priv_key_uri.

* Adds helper factory method to create an SpxKey instance from raw bytes.

* Adopts changes in usage example and tests.