routes/suite.py: add /default endpoint

[Devansh3712]

Evaluation task for Google Summer of Code 2024

• Setup teuthology-api and share screenshot that you're able to schedule runs from teuthology-api (i.e. share screenshot of success response for /suite endpoint)

• Open a PR for adding a new endpoint to teuthology-api for suite command defaults: scheduling page: populate default values using new teuthology-api endpoint pulpito-ng#41
• [optional] Add unit tests for the new endpoint added above
• Understanding of current build-and-schedule workflow and new auto-schedule workflow in proposal

Checklist

• Changes tested in container setup
• Changes tested in non-containerized setup
• Includes tests
• Documentation updated