Use config.yaml file from default cache location, if it exists

[nkoenig]

🎉 New feature

Summary

This reads the config.yaml or config.yml file from the cache directory when the ClientConfig is created. Without this change, a user needs to enter the -c command line argument each time or manually call the LoadConfig function.

Test it

1. Create an API key from app.gazebosim.org
2. Create a config.yaml file in ~/.gz/fuel that contains:

---
servers:
  -
    url: https://fuel.gazebosim.org
    private-token: YOUR_API_KEY

3. Download a private model with something like gz fuel download -u https://fuel.gazebosim.org/1.0/OWNER/models/MODEL.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[nkoenig]

Tests are failing due to the fuel server instance.