storage: Add proxy storage with retry policy

This implements the actual behavior the loader-core (dvcs ones) exposes. This will allow to configure other loaders with the same retry (currently limited to content_add here for discussion purposes [1]).

This should allow to:

• take care of issues like [2] (hash collision in package loaders otherwise taken care of in dvcs loaders)

• clean up some more the current loader-core (original loader-core wraps the storage's entrypoints with send_{content,origin,...} methods which adds that retry policy) [3]

• [1] I will complete the implementation if we agree to continue in that direction

• [2] https://sentry.softwareheritage.org/share/issue/d4f1208b7eec4b43b11e38494ff039cc/

• [3] https://forge.softwareheritage.org/source/swh-loader-core/browse/master/swh/loader/core/loader.py$331-361

Test Plan

tox

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Migrated from D2511 (view on Phabricator)