identifiers: support optional contextual parts for line numbers and origin

We have discussed (yours truly, @rdicosmo , and @anlambert) how to add to our persistent identifier scheme optional parts that denote the context of interest of the identified object. In particular we have discussed adding an optional origin, and optional line numbers.

The desired syntax is the following, where square brackets denote optional "qualifiers" (piggybacking on the ARK identifier terminology):

swh:1:…[/Lnn[-mm]][/ORIGIN]

where "L" and "-" are concrete syntax, ORIGIN an URI starting with a scheme, "nn" and "mm" are integer numbers.

This should be added to the persistent identifier documentation and supported by the web app resolver.

Migrated from T1040 (view on Phabricator)

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