Add GraphQL schema generation script and seed schema file

[shangyian]

Summary

This PR adds a script to save the GraphQL schema used by the DataJunction server into a .graphql file, and seeds the initial generated schema. This supports downstream consumers that rely on a stable .graphql schema file.

Changes include:

• A Python script (scripts/graphql_schema.py) that generates the schema from the Strawberry GraphQL app and writes it to a graphql file.
• The initial generated schema at datajunction_server/api/graphql/schema.graphql.
• A pre-commit hook to ensure the schema stays up-to-date

Test Plan

Locally

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

N/A

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[agorajek]

Cool!