Feature, Public API | Enable SqlDataRecord support for TVPs with computed columns

[edwardneal]

Description

The MS-TDS spec supports a client recognising that a field in a table-valued parameter might well be computed by the server. An example of a TVP which does this is:

CREATE TYPE [dbo].[MyType] AS TABLE
(
    [Value1] INT NOT NULL,
    [Value2] INT NOT NULL,
    [Added] AS ([Value1] + [Value2])
)

Under such circumstances, the client can set the fComputed bit in the TVP_COLMETADATA definition. We also need to set the fDefault bit - without setting this, SQL Server expects the client to send a value for the computed column, and throws an exception indicating that I'm trying to overwrite a computed column with a value when we send one.

Once the fComputed bit is set, the computed columns in the TVP are populated by SQL Server as we'd expect.

Enabling the client to set this necessitates a public API change of some form. In this case, I've added an IsComputed property to SqlMetaData:

public sealed class SqlMetaData
{
    public bool IsComputed { get; init; }
}

This is a slightly different approach to the public API than the current set of constructors, but I think it's the most practical one - I think most of the constructors would want to be able to specify whether the field is computed, and we'd essentially double the number of constructors if adding a separate parameter to all of them.

I'm leaving this PR in draft until there's time for the public API to be reviewed. Once we're happy with it, I'll adjust the API surface accordingly.

@mdaigle - this intersects with some of the work you'd been doing on the issue and provides a fix for SqlDataRecord.

Issues

Fixes #3242. It doesn't introduce anything to enable DataTable- or DbDataReader-based support, but I don't think there's a way to enable that support client-side.

Testing

New manual tests have been added, and all pass.

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