Add a serialized_data schema type

Problem/Motivation

Follow-up from #1031122: postgres changeField() is unable to convert to bytea column type correctly →

Drupal 6 and possibly some contrib modules stored serialized data in a varchar, this is not recommended, and particularly breaks on postgres.

Proposed resolution

Add a serialized_data schema type to make it clear that the column stores serialized data. This would be mainly for documentation purposes and would map to blob/bytea.

Remaining tasks

User interface changes

API changes

Addition only, although also a new best practice.

Data model changes