Switch package to use fsspec for backends

[hagenw]

Closes #230

This replaces our own backend implementations by supporting fsspec, which provides implementations for several different backends. I flattened the interface module and moved all classes to the top of the package.

As there exist no fsspec compatible implementation for Artifactory, I would propose we wait with this pull request until we no longer need to support Artifactory.

TODO:

• owner() is not supported by fsspec
• Add support for relative + absolute file path as fsspec supports it, e.g. fsspec.LocalFileSystem

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.0%. Comparing base (937da89) to head (c15a569).

Additional details and impacted files

Files with missing lines	Coverage Δ
audbackend/__init__.py	100.0% <100.0%> (ø)
audbackend/core/base.py	100.0% <100.0%> (ø)
audbackend/core/conftest.py	100.0% <100.0%> (ø)
audbackend/core/errors.py	100.0% <ø> (ø)
audbackend/core/maven.py	100.0% <100.0%> (ø)
audbackend/core/unversioned.py	100.0% <100.0%> (ø)
audbackend/core/utils.py	100.0% <ø> (ø)
audbackend/core/versioned.py	100.0% <100.0%> (ø)