Drift 2.12.0

This drift release adds support for defining a database index in Dart and improves JSON support.

Full list of changes:

• Add support for table-valued functions in the Dart query builder.
• Support the json_each and json_tree table-valued functions.
• Add the @TableIndex annotation for table classes to add an index to the table.
• Add the TypeConverter.json method to define type converters storing JSON values more easily.
• Add TypedResult.readWithConverter to read a column with a type converter from a join result row.