Make masking_admin and blocking_admin regular swh backends

This allows to use swh db tooling with e.g. either the 'storage:masking' module or the 'storage_masking_admin:postgresql' one. This later one is declared as an alias for the "true" storage backend (aka "storage:masking"). Note that the db tooling will look for the db connection in the actual given config section, not the aliased one. That means that a command like swh db init storage_masking_admin:postgresql will use the db entry in the masking_admin config section.

Depends on swh-core!414

Jenkins job DSTO/gitlab-builds #903 failed in 3 min 47 sec.
See Console Output, Blue Ocean and Coverage Report for more details.

Notes:

• blocking and masking proxies are now declared as backends in pyproject.toml
• there should be no need for db migration here, the registered 'dbmodule' stays 'storage:masking' when initializing it using 'swh db init storage_masking_admin'
• BUT the configuration now expects the 'masking_admin' section to be 'storage_masking_admin' (the name of the backend category); it would be nice to add a bw compat safety mesh for this...

added 5 commits

• 63dbddec...58b39b24 - 3 commits from branch swh/devel:master
• aa44be8f - Improve docstrings of several storage backends
• ab2ff476 - Make masking_admin and blocking_admin regular swh backends

Compare with previous version

Jenkins job DSTO/gitlab-builds #907 failed in 4 min 9 sec.
See Console Output, Blue Ocean and Coverage Report for more details.