graphql: T4753: generalize system_status to composite_{query,mutation}

[jestabro]

Change Summary

For switching between REST-style key and token-based authentication methods, all schema definition files need to be automatically generated (as 'key' is defined in the schema itself). SystemStatus was the lone non-generated case; generalize to 'composite' query/mutation, a category that will be enlarged or dropped, as use cases require.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes)
• [ ] Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• [ ] Other (please describe):

Related Task(s)

• https://phabricator.vyos.net/T4753

Component(s) name

http-api

Proposed changes

How to test

The request query 'SystemStatus' will show no change in response.

Checklist:

• [X] I have read the CONTRIBUTING document
• [X] I have linked this PR to one or more Phabricator Task(s)
• [X] I have run the components SMOKETESTS if applicable
• [X] My commit headlines contain a valid Task id
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly