[11.x] Introduce method `Blueprint::rawColumn()`

[Jacobs63]

This PR introduces a new column method when creating/updating table schema.

Currently, when trying to create columns which are not natively supported by any of the grammars, we have to instead use database statements. This creates a rather big inconvenience as we cannot use a single table creation callback, because database statements are fired immediately and therefore cannot be used in Schema::create(<table>, <callback>).

Imagine a scenario, where we would like to create a:

1. Table named posts
2. Containing auto-increment column id
3. Containing an integer column of display width 1 named legacy_boolean with a default value of 0
4. Containing a composite index for these two columns

The example code to solve this would likely be as follows:

new class extends Migration {
    public function up()
    {
        Schema::create('posts', function (Blueprint $table) {
            $table->id();
        });

        DB::statement('alter table `posts` add `legacy_boolean` int(1) default 0 not null');

        Schema::table('posts', function (Blueprint $table) {
            $table->index(['id', 'legacy_boolean']);
        });
    }
};

This PR introduces a new column creation method which supports directly specifiying the column type definition used for creating a column:

new class extends Migration {
    public function up()
    {
        Schema::create('table', function (Blueprint $table) {
            $table->id();
            $table->rawColumn('legacy_boolean', 'int(1)')->default(0);
            $table->index(['id', 'legacy_boolean']);
        });
    }
};

This however has one possible drawback - this is not grammar-agnostic and therefore would be specific to the database driver used, even though, by-default, working with columns is grammar-agnostic as the underlying column definitions are compiled by the grammar implementation used by the driver. On the other hand, the previous solution using database statements is also not grammar-agnostic and therefore we're just moving the drawback from one place to another, meaning, this should not be an issue after all, as we were already facing it elsewhere.

[taylorotwell]

I think rawColumn would be a more fitting name as @morloderex notes. Please mark as ready for review when the requested changes have been made.

[Jacobs63]

Not sure that I'm a fan of using the callback as I feel like it doesn't really have added value. The connection is already accessible via DB::connection().

Requested changes have been applied, feel free to re-check.