Initial version of the swh.objstorage.replayer package

this consists in the copy and adaptation of content replayer related files
from swh.journal v0.0.31.

Notable modifications:

• test_write_replay.py has been merged into test_replay.py,
• any reference to swh.journal.replay has been replaced,
• adapted cli module to make added replayer commands subcommands of the objstorage's main cli group,
• drop usage of vcversionner in favor of setuptools_scm.