Per-backend column type overrides, e.g. json on mysql / jsonb on postgres

Motivation

Column types are not portable across databases, and today SeaORM/sea-query can only express a single fixed column type per column. Two concrete consequences:

• The common "JSON" column needs json on mysql and jsonb on postgres. There is no single column definition that produces the native right type on both; you either pick one or work around it outside the entity.
• Types that only some databases have (tinyint, longblob, postgres interval, ...) force either a custom ColumnType::Custom("...") string that is wrong on every other backend, or a bigger portable type. Long-standing issues #426 (interval), #631 (longblob), #213 (DATE) and the newer composite/domain type requests (#2895, #2863) all sit around this gap.

Proposed Solutions

I'd like to start a discussion on a per-backend column type override, roughly:

ColumnDef::new(Column::Payload)
    .string() // abstract fallback for backends without an override
    .per_backend(|backend| match backend {
        Backend::MySql => ColumnType::Json,
        Backend::Postgres => ColumnType::JsonB,
        _ => ColumnType::String(None),
    })

(or any equivalent shape — a map from backend to ColumnType, an extension of ColumnType::Custom, or derivable helpers; the ergonomics are for maintainers to steer).

The semantics that have proven themselves in other ORMs: the backend-specific type wins on that backend, other backends fall back to the abstract type, and columns without overrides render exactly as today.

• ent (Go) has field.String("s").SchemaType(map[string]string{dialect.MySQL: "varchar(255)", dialect.Postgres: "text"}) as a built-in
• SQLAlchemy has String(255).with_variant(VARCHAR(255, charset="utf8"), "mysql")
• gorm recently merged dialect-qualified type tags (go-gorm/gorm#7867)

Happy to collaborate on a design and implementation if maintainers think this fits sea-query/SeaORM's direction.

Additional Information

The same need shows up across ecosystems; the ent and SQLAlchemy links above are the closest prior art. No behavior would change for existing entities — the fallback path is the current rendering.