hashutil: Add support for md5 sum

Enable to compute md5 sum through the hashutil.MultiHash class.

Nevertheless, md5 is not put in DEFAULT_ALGORITHMS set and must be explicitely requested by client code.

Related to swh/infra/sysadm-environment#2400 (closed) Related to swh-loader-core!247 (closed)

Depends on !205 (closed)

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Migrated from D6755 (view on Phabricator)