Shorthand to grab the `Query` Object from the schema

**Is your feature request related to a problem? Please describe.**
Not a problem, but would reduced the need for unnecessary code.

Essentially, when playing around with a subgraph's schema, it is often useful to grab the `Query` GraphQL object which contains all toplevel fields that can be selected from the API. Currently, one has to go through the `SchemaMeta.type_map` dictionary, of which the type of the values is a union of all possible GraphQL types.

However, we know that `Query` will always be an `ObjectMeta` type. Moreover, we know that all GraphQL APIs have a `Query` object (it's part of the GraphQL specifications). Therefore, it would make sense to have a little helper for this.

**Describe the solution you'd like**
```python
schema: SchemaMeta = ...

# Instead of this
query_objectmeta = cast(TypeMeta.ObjectMeta, schema.type_map["Query"])

# This! query_objectmeta is correctly typed with TypeMeta.ObjectMeta
# SchemaMeta.query_object would be a property
query_objectmeta = schema.query_object
```

**Describe alternatives you've considered**
None. Apart from using a different name for the helper property (e.g.: `query_objectmeta`, or simply `query`)

**Additional context**
The same arguments could also be made for having helpers for the `Subscription` and `Mutation` objects, but these are not guaranteed to be present in a GraphQL API.

**Implementation checklist**
- [ ] Add `SchemaMeta.query_object` helper property

Look at the SchemaMeta class in the codebase to understand its structure and the type_map dictionary. The helper property should be added to SchemaMeta, likely in the same file. Check if there are existing tests for SchemaMeta to understand how to test the new property. The change is small and localized, focusing on adding a property that returns the 'Query' object from type_map with proper typing.

Scritto dal modello di indicizzazione a partire dal testo della issue.