[typed_sql] JSONB/JSON support

[jonasfj]

• [x] https://github.com/google/dart-neats/pull/291
• [x] Support for @DefaultValue(JsonValue{'foo': 42, 'bar': null})) on fields with JsonValue.
• [x] Tests for .distinct() with JsonValue.
• [x] Expressions and tests for JsonValue
  • [x] .orElse and .orElseValue
  • [x] .isNull, .isNotNull.
    • We need careful documentation that explains the different between NULL in SQL and null and JSON, at-least for operators that can work on Expr<JsonValue?>.
    • Including tests in test/typed_sql/expr/.
  • [x] JSON specific operators, like .containsKey, .getKey(), .length`, etc.
• [x] Forbid use of JsonValue in:
  • [x] @PrimaryKey,
  • [x] @ForeignKey and @References
  • [x] @Unique constraints.
• [x] Test use of JsonValue in .groupBy
• [x] Test JsonValue? as field, see test/typed_sql/nullable/
• [x] Add support and tests for final class MyDataType implements CustomDataType<JsonValue>
  • [ ] Forbid implementation of CustomDataType<JsonValue> and Comparable<T>.
• [x] Update documentation guides to mention JsonValue.
• [ ] Add dedicated documentation page with detailed example for JsonValue.
  • How to use JsonValue
    • Define a JSON field in a schema.
    • Declaring a default value.
    • Caveats when using distinct/group_by with JsonValue (possible difference between databases).
    • Using operators, and caveats around SQL NULL and JSON null. (Strongly recommend not using JSON null, if feasible).
    • Operators on Expr<JsonValue>, how to use, and caveats.
  • How to subclass using CustomDataType<JsonValue>
  • How to add custom operators on Expr<CustomDataType<JsonValue>> using JSON operators.

@isoos