SQL Server: Allow switching to the new JSON type by default

[roji]

Although we've added experimental support for the new Azure SQL JSON type, it must currently be opted into for each and every owned type/primitive collection. We should have some way of allowing the user to opt into the JSON type everywhere instead.

Note: when the new JSON type goes out of preview on Azure SQL, we can consider mapping to it by default when the user configures EF with UseAzureSql(). This would be a be a breaking change, we'd need to consider when the migration would look like (e.g. does/will Azure SQL support simply altering the type from nvarchar(max) to json in our migrations?).

/cc @ajcvickers