Migrations: Defer type mapping

[bricelam]

Like in EF6, we should defer type mapping until migration apply time. For example:

migrationBuilder.CreateTable(
    name: "MyTable",
    columns: table => new
    {
        Id = table.Column<int>()
    });

migrationBuilder.AlterColumn<int>(
    name: "Id"
    table: "MyTable");

migrationBuilder.AddColumn<string>(
    name: "Name"
    table: "MyTable");

migrationBuilder.CreateSequence<long>("MySequence");