Enable support for marking stack and context variables as secret via GraphQL

When creating context or stack variables via GraphQL, marking them as secret currently requires manual action in the UI. It would be beneficial to have the ability to mark them as secret directly through GraphQL.

https://docs.spacelift.io/concepts/configuration/context.html#editing-environment-variables

https://graphdoc.io/preview/mutation/contextConfigAdd?endpoint=https://demo.app.spacelift.io/graphql