Ensure foreign key columns create an index by default and add tests for constrained method behavior

[theparadoxShin]

This PR fixes the bug mentionned in issu #56266 , enabling web artisans using PostgreSQL to work more quickly without worrying about whether an index has been created or not. All cases of index creation for good performance are handled.

[mathiasgrimm]

I've created a draft PR that is related to this

[theparadoxShin]

@mathiasgrimm A lot of configuration I think . I took the road of simplicity by creating automatically index on foreign key by default !

And I saw that you created a new function addAutoForeignKeyIndexes() yet addFluentIndexes() exists .

But I like the idea to say to the framework that you don't want index on a particular faoreign key so I will keep the withoutIndex() , and it can look like this in real implementation : " $table->foreignId('user_id')->constrained()->withoutIndex(); "

Personnaly on my solution I will update the constrained() function like this to add a global configuration :

if (! isset($this->index)) {
    // Verified the config if exist but true by default so we create 
    $autoCreate = $this->blueprint->getConnection()
        ->getConfig('foreign_key_implicit_index_creation', true);
    if ($autoCreate) {
        $this->index = true;
    }
}

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!