Update spec for new staging areas for linking updates and deletes

[aaclan-ebi]

Update spec according to what has been agreed on last post DCP2 demo meeting

Related to ebi-ait/dcp-ingest-central#695
Related slack thread https://humancellatlas.slack.com/archives/C01360XN04S/p1646223040031019

Notes:

• New staging area enum types - normal, delta, updated (or latest tbd)
• "updated" staging areas will always contain the latest set of metadata, links, descriptor files
• At some point, we may want to add support for updated data files and in that case, there will be data files present in the staging area
• Sequence diagram: https://collabuml.com/update-links

A new staging area which we will require Data Import to reset a project and import the staging area like a new project.
Pro(s): The design is simpler and more straightforward and covers all scenarios for metadata updates and deletes.
Con(s): It may be expensive for an extremely large dataset (e.g. Tabula Muris), in which case we could use the delta staging area approach once we really need to.