Allow expression row value comparisons via extension methods

[roji]

See dotnet/efcore#26822 - this implements that but for PG only.

Most databases allow comparing row value expressions:

WHERE (a, b) > (3, 4)
-- Equivalent to:
WHERE a > 3 OR (a = 3 AND b > 4)

This is is notably useful for keyset pagination with multiple keys (see this detailed presentation).

I prevently implemented #2111 and reverted a bad approach, which pattern-matched the expanded form above in the tree, and replaced it with the row value expression. This was both brittle (some tree shapes failed to be matched), and also results in complicated LINQ queries.

Ideally, users would be able to express row value comparisons in LINQ queries by using value tuples (see dotnet/efcore#26822):

ctx.Blogs.Where(b => EF.Functions.GreaterThan((b.X, b.Y), (3, 4)))

However, since value tuple syntax isn't yet supported in C# expression trees, we can allow object arrays instead:

ctx.Blogs.Where(b => EF.Functions.GreaterThan(new object[] { b.X, b.Y }, new object[] { 3, 4 }))

/cc @mrahhal

[roji]

OK, I ended up going in a slightly different direction, and using ValueType<T1, T2...>(x, y...) instead of arrays to express row values in LINQ:

_ = await ctx.Customers
    .Where(c => EF.Functions.GreaterThan(
        new ValueTuple<string, string>(c.City, c.CustomerID),
        new ValueTuple<string, string>("Buenos Aires", "OCEAN")))
    .CountAsync();

This is a bit uglier/more verbose, but:

• Tuples really are the appropriate .NET construct for mapping a row value.
• When some day ValueTuple syntax becomes supported in LINQ expression trees ((x, y)), this should transition seamlessly.
• Doesn't rely on provider array support which is PG-specific, so should be easily portable to the general relational layer if we want.
• Avoids any confusion with arrays, which are also supported in PG. For example, arrays can also be compared in PG (very different thing from row value comparisons).
• We can selectively make NewExpression for ValueTuple non-evaluatable, rather than NewArrayExpression, which is used in other contexts where it can be evaluatable (see Support for row value comparisons #2350 (comment)).

/cc @smitpatel

[roji]

Docs: npgsql/doc@511a6af in the 7.0 branch.