Metadata work for json.

[maumar]

Separated from main PR to make the review easier.

This PR includes:

• builder methods,
• conventions,
• relational model,
• model validation,
• migrations,
• update.

[roji]

Here's a 1st round of comments from me. There are some questions we may want to discuss in design (e.g. distinction between columns and JSON properties, whether JSON properties should be represented as columns in the relational model).

[bricelam]

Migrations changes LGTM

[AndriySvyryd]

Add a JSON column to ModelSnapshotSqlServerTest

[AndriySvyryd]

Add a test to MigrationsModelDifferTest for converting a string property of JSON type to a JSON-mapped entity type

Stale