Migrations: Support database schemas

[anpete]

Created a relational model and tried to used a custom, non-existent db schema. Database creation failed because it looks like migrations has no support for creating database schemas.

[ghost]

commit e3a3901

[bricelam]

Re-opening. I've regressed this.

[rowanmiller]

When you work on this the ModelDiffer currently throws if you add a schema since it tries to create an operation to move from an unspecified (and therefore null) schema.

System.ArgumentNullException: Value cannot be null.
Parameter name: newSchema
at Microsoft.Data.Entity.Utilities.Check.NotEmpty(String value, String parameterName)
at Microsoft.Data.Entity.Relational.Migrations.Operations.MoveTableOperation..ctor(String name, String schema, String newSchema, IReadOnlyDictionary`2 annotations)
at Microsoft.Data.Entity.Relational.Migrations.Infrastructure.ModelDiffer.d__15.MoveNext()

[bricelam]

Notes from today's design meeting:

• When a new schema is encountered in the model differ, we'll scaffold a CreateSchema operation
• CreateSchema will create the schema if it doesn't already exists
• We'll never scaffold DropSchema, but it can manually be added to the migration

[smitpatel]

https://github.com/aspnet/EntityFramework/blob/dev/test/EFCore.CrossStore.FunctionalTests/TestModels/CrossStoreContext.cs#L23