Use extension codecs strategy for groups of common metadata

[jisantuc]

Improvement

#48 introduced the idea of providing decoders for extensions that users could have applied if they care. We should do a similar thing for each group in common metadata fields.

Notes + Context

The imagined end-user is Franklin, in which users query for items that provide a certain common metadata group (in extensions? I don't know, it's not clear how we make that happen), and we validate with the common metadata group codec at the time that we serialize it to the user.