Onboarding - Add Jitsu key to project GraphQL type

Proposal

The frontend is going to need to retrieve the Jitsu key from the projects settings to render the key in the instrumentation instructions.

We should add this key to https://docs.gitlab.com/ee/api/graphql/reference/#project in the same way that mergeCommitTemplate has been added.

Implementation plan

1. For GitLab Premium, with the :cube_api_proxy feature flag enabled, return the jitsu_key from project settings if it is set.
2. For everyone else, return nil.
3. Add the field to the project_type for GraphQL.
4. Delegate retrieval of the jitsu_key to :project_setting.
5. Update specs as required.