fix: allow query to be passed as instance

[alancolant]

Summary

PR: Allow Using a Query Instance in Addition to a Class-String

This PR adds the ability to use a Query instance in addition to a class-string by default.

Before:

Previously, if I built my schema at runtime like this:

$config = [
    'types'     => [/*...*/],
    'mutations' => [/*...*/],
    'query'     => [new FindQueryForModel(User::class)]
];

$schema = GraphQL::buildSchemaFromConfig($config);

I would get the following error:
Cannot use object of type FindQueryForModel as array

Changes:

• I have added tests.
• I have handled instantiated Field objects by adding a specific case in GraphQL.php -> buildObjectTypeFromFields:

elseif ($field instanceof Field) {
    $field = $field->toArray();
}

Documentation:

I have not documented this possibility in README.md because it would be a bad practice to use this in the config file. Doing so would break Laravel's caching mechanism, as this type is not serializable.

---

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist:

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md (Not needed)
• Code style has been fixed via composer fix-style