fix: generated type of PostgreSQL's interval type in pgx

[d-tsuji]

Fixes #1224

When the driver is github.com/jackc/pgx, it is correct to use pgtype.Interval.

[d-tsuji]

@kyleconroy
I have checked past discussions and only generate as pgtype.Interval type if you are using pgx.

#429

The reason why the interval type in PostgreSQL is pgtype.Interval type is due to the following code

https://github.com/jackc/pgtype/blob/72948465f0b9bf2f74e147370d7b61a7f3726cf0/pgtype.go#L299

ci.RegisterDataType(DataType{Value: &Interval{}, Name: "interval", OID: IntervalOID})

We believe this is sufficient for many use cases. It improves the development experience by allowing users to query without setting overrides.

Are there any other concerns? And is there anything else I can help you with?