specify a manifest format for documenting archived software

To support the use case of scientists willing to document the software relevant for a given paper, we want to have a manifest format capable of fully describing some software archived by Software Heritage.

In its essence, a manifest should point to Software Heritage objects using the URI scheme of #335 (closed). But then, different level of details are possible:

(minimum detail) just point to a manifest file archived somewhere (possibly on SWH), using a manifest ID
point to a directory
fully describe the directory content, with a pair <pathname, content id> for each file
(maximum detail) as above + a revision history, describing each revision in full ← this will be crazy-large for long histories

In addition to the above, various kinds of metadata could be added:

SWH-specific metadata: when and where the archived code has been found
user-provided metadata, submitted to SWH at the time of ingestion request (e.g., Dublin Core, paper references, etc.)

Migrated from T337 (view on Phabricator)

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