Task: Provide GraphQL gateway URL

[andreaskienle]

At the moment, the GraphQL gateway URL is hardcoded:

• frontend-config.json
• graphql.config.yaml for type generartion

ToDos:

• Adjust frontend-config in our landscape
• Consider reading the URL from frontend-config.json in graphql.config.yaml if necessary: https://the-guild.dev/graphql/codegen/docs/advanced/programmatic-usage

[andreaskienle]

Could maybe done together with #79