Add GraphQL support

[erikgaas]

Adds a new graphql module with:

• GhGql class - lazy-loaded GraphQL client with schema introspection
• list_queries() - list available query types
• query_args(name) - show arguments for a query
• type_fields(name) - list fields on a type
• gh_query(query, vars) - execute queries

Schema is lazy-loaded (not at import time) so importing won't block.

Example cells use #| eval: false to skip during CI since they require a token.