Read DATE as DateOnly value

[bgrainger]

For backwards-compatibility reasons, MySqlDataReader.GetValue will return a DateTime object for DATE columns, even though this could be represented by a .NET DateOnly struct.

For users who want a DateOnly, there are two simple workarounds:

• MySqlDataReader.GetFieldValue<DateOnly>
• MySqlDataReader.GetDateOnly

However, there may be cases such as PomeloFoundation/Pomelo.EntityFrameworkCore.MySql#1836 where it would be useful for the "natural" type of the column (retrieved by GetValue) to be DateOnly so no explicit conversion has to take place.

Since there is already a connection string option (AllowZeroDateTime) that causes GetValue to return MySqlDateTime (instead of DateTime), it's possible that a new connection string option could be introduced that switches the behavior of GetValue for DATE columns to return DateOnly. (Perhaps over time this could even become the default behavior and users would have to opt out of the new behavior instead of opting in.)

We could also consider returning TIME columns as a TimeOnly type instead of TimeSpan, but that would only work if the user was deliberately limiting values stored in that column to the range 00:00:00 to 23:59:59.9999999 instead of the full range of the TIME type, which is -838:59:59.000000 to 838:59:59.000000. This seems like it would always need to be an opt-in feature.

See also #963 (comment).

[iamcarbon]

Would it make sense to add a similar option to read JSON columns as byte[] (instead of a string)? This would allow the column to be bound to a JsonObject / JsonElement without additional conversions / allocations.

[bgrainger]

Another existing request for DATE columns to be returned as DateOnly: #1231 (comment)

[bgrainger]

@iamcarbon Created #1454 for this idea. Note that MySqlDataReader.GetStream provides a Stream over the existing in-memory data without allocating a new byte[] (although it does of course allocate a Stream object).