SWHID v2

This is a meta-task to track activities related to rolling out the next version of SWHID (aka v2). This includes the following:

• complete on paper spec
• aligned with work done on new git hashes
• including migration plan from/cohabitation with v1 (N.B.: we need to maintain SWHID v1 support forever anyway)
• understand impact on internal microservice architecture (related to swh/devel/swh-web#1805, in particular use SWHIDs everywhere (core SWHIDs, without qualifiers))
• keep correspondence with v1 (there may be multiple v2 for one v1!)
• reviewed by crypto experts

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Migrated from T3134 (view on Phabricator)