Feature/schema from existing graph

[stellasia]

Description

This PR adds a new component: SchemaFromExistingGraphExtractor that builds ta GraphSchema object from an exiting database:

    with neo4j.GraphDatabase.driver(
        URI,
        auth=AUTH,
    ) as driver:
        extractor = SchemaFromExistingGraphExtractor(driver)
        schema: GraphSchema = await extractor.run()
        # schema.store_as_json("my_schema.json")
        print(schema)

It leverages the get_structured_schema function and transform its output into a GraphSchema object.

Type of Change

• New feature
• Bug fix
• Breaking change
• Documentation update
• Project configuration change

Complexity

Complexity: Low

How Has This Been Tested?

• Unit tests
• E2E tests
• Manual tests

Checklist

The following requirements should have been met (depending on the changes in the branch):

• Documentation has been updated
• Unit tests have been updated
• E2E tests have been updated
• Examples have been updated
• New files have copyright header
• CLA (https://neo4j.com/developer/cla/) has been signed
• CHANGELOG.md updated if appropriate