Create.UniqueConstraint API should be as feature complete as Create.Index for creating unique indexes

[jzabroski]

See #1081 (review)

At a high level, SQL Server only supports creating unique constraints with included column lists and filter clause using the CREATE INDEX syntax. The goal is to make it so that Create.UniqueConstraint offers the same expressiveness as Create.Index().OnTable().Column().Unique(): filtered indexes, included column lists.

• For SQL Server, the SqlServer2005Generator has an ABI break: public virtual string GetIncludeString(CreateIndexExpression column) has the parameter renamed to public virtual string GetIncludeString(CreateIndexExpression expression). This is considered a minor break, and unexpected that clients would call this method through reflection.
• For SQL Server, the extension methods for Create Constraint should no longer return void and return ICreateConstraintOptionsSyntax instead:
  • Clustered:
    • was: public static void Clustered(this ICreateConstraintOptionsSyntax expression)
    • now: public static ICreateConstraintOptionsSyntax Clustered(this ICreateConstraintOptionsSyntax expression)
    • what this enables: Not much. If you write createConstraintExpression.Clustered().Include("Column1"), it will fail, since it is non-sensical to include a column in a clustered index. If you write createConstraintExpression.Clustered().Filter("Column1 IS NOT NULL"), it will fail, since it is not possible to filter a SQL Server clustered index. The benefit of this API is it makes it easier for the user to refactor their code by simply changing createConstraintExpression.Clustered() to createConstraintExpression.NonClustered().
  • NonClustered:
    • was: public static void NonClustered(this ICreateConstraintOptionsSyntax expression)
    • now: public static ICreateConstraintOptionsSyntax NonClustered(this ICreateConstraintOptionsSyntax expression)
    • what this enables: Previously, you could only write: Create.UniqueConstraint("UQ_Foo").OnTable("Foo").Column("Bar").Filter("BAR IS NOT NULL").NonClustered();. Now, you can also write: Create.UniqueConstraint("UQ_Foo").OnTable("Foo").Column("Bar").NonClustered().Filter("BAR IS NOT NULL");
• Helper extension methods API breaking change:
  • Rename SqlServerExtensions.Include to SqlServerExtensions.CreateIndexInclude
  • Added SqlServerExtensions.CreateUniqueConstraintInclude

Additional Considerations

• CreateIndexExpressionNonKeyBuilder should remove public ICreateIndexNonKeyColumnSyntax Include(string columnName) in a future release? It's a question. It's relying on SupportAdditionalFeatures, which is a bit weird given the implementation points to SqlServerExtensions.CreateIndexInclude