feat: allow conditionally setting eager load

[JaZo]

I'd like to conditionally allow/disallow eager loading, just like you can conditionally hide/show a field. A simple boolean would be enough in my use case, but to make it more in line with hidden and readOnly I've also added closure support. The only difference is that these closures receive a request and the new one doesn't as I couldn't find a good way to get the request.

N.B. I had first added the parameter to the existing cannotEagerLoad method, but that was pretty confusing due to the double negation.

Before

/**
 * @return \LaravelJsonApi\Contracts\Schema\Field[]
 */
public function fields(): array
{
    $userField = BelongsTo::make('user');
    if (Gate::denies('viewUser', Blog::class)) {
        $userField->cannotEagerLoad();
    }

    return [
        ID::make()->uuid(),
        $userField,
    ];
}

After

/**
 * @return \LaravelJsonApi\Contracts\Schema\Field[]
 */
public function fields(): array
{
    return [
        ID::make()->uuid(),
        BelongsTo::make('user')->canEagerLoad(Gate::allows('viewUser', Blog::class)),
    ];
}

[lindyhopchris]

Thanks for the PR, this looks like a good thing to add.

There's just one suggested change.