Add an option to ignore default index

[pillo79]

Describe the request

The CLI always automatically pulls the latest package_index.json from the web, and uses that in all requests (core search etc).
For CI purposes, it would be helpful to add an option to allow it to only load a specific set of URLs (for example, the files just being signed) to check that only their contents are reported.

Describe the current behavior

The CLI always loads the main package_index.json. The current workaround described in #2529 will break without warning.

Arduino CLI version

0.35.2

Operating system

N/A

Operating system version

--

Additional context

No response

Issue checklist

• I searched for previous requests in the issue tracker
• I verified the feature was still missing when using the nightly build
• My request contains all necessary details