[9.x] Add unique deferrable initially deferred constants for PostgreSQL

[markuskooche]

Currently it is not possible to check a unique constraint only after updating.

For this, DEFERRABLE INITIALLY DEFERRED is needed.

create table pictures
(
    id bigserial primary key,
    ...
    constraint pictures_post_id_post_type_sequence_unique
        unique (post_id, post_type, sequence)
            deferrable initially deferred
);

You can easily apply this to a foreign id with the following function:

$table->foreignId('foo')->deferrable();

This works because it is already observed in the foreign id.
https://github.com/laravel/framework/blob/9.x/src/Illuminate/Database/Schema/ForeignKeyDefinition.php#L8-L9

However, not with the UNIQUE Keys.
https://github.com/laravel/framework/blob/9.x/src//Illuminate/Database/Schema/Grammars/PostgresGrammar.php#L146-L160

This problem has already been listed in discussion #40953.

Now you can also put a DEFERRABLE INITIALLY DEFERRED or a DEFERRABLE INITIALLY IMMEDIATE on unique keys.

DEFERRABLE INITIALLY DEFERRED:

$table->unique(['post_id', 'post_type', 'sequence'])
    ->deferrable()
    ->initiallyImmediate(false);

DEFERRABLE INITIALLY IMMEDIATE

$table->unique(['post_id', 'post_type', 'sequence'])
    ->deferrable()
    ->initiallyImmediate();