[5.8] Add Postgres support for collation() on columns

[danmichaelo]

The MySQL and SQL Server schema builders already had support for setting collation on columns using the collation() methods. This PR adds Postgres support for the same functionality. Postgres have had support for collated columns since version 9.1 released in 2011.

Schema::create('users', function (Blueprint $table) {
    $table->string('name')->collation('nb_NO.utf8');
});

I think could go safely onto 5.8?