Additional resources:
https://doc.wikimedia.org/Wikibase/master/php/docs_topics_datatypes.html

Technical notes:

• implement “data type aware” value deserialization
  • consider optimizing this right away so that it only looks up the data type if necessary. How to tell whether it's necessary: look up all data types corresponding to the given value type. The lookup is necessary only if one of the data types register a custom parser-factory-callback.
• be extra careful when "plugging in" the new type of deserializer that does the additional lookups. We don't want to accidentally put this live before testing and/or optimizing it.
• document this in an ADR
• document the new field in extensions/Wikibase/docs/topics/datatypes.md