feat: add support for plugin index

[knqyf263]

Description

This PR adds support for the plugin index. See #6641 for more details.

Usage

1. Download the plugin list:

$ trivy plugin update

2. Discover plugins available on the index:

$ trivy plugin search
NAME                 DESCRIPTION                                                  MAINTAINER           OUTPUT
aqua                 A plugin for integration with Aqua Security SaaS platform    aquasecurity
kubectl              A plugin scanning the images of a kubernetes resource        aquasecurity
referrer             A plugin for OCI referrers                                   aquasecurity           ✓
[...]

3. Choose a plugin from the list and install it:

$ trivy plugin install referrer

4. Use the installed plugin:

$ trivy referrer --help

5. Keep your plugins up-to-date:

$ trivy plugin upgrade

6. Uninstall a plugin you no longer use:

trivy plugin uninstall referrer

Schema update

This PR also adds changes to plugin.yaml.

• Rename usage to summary for clarity
  • Trivy also recognizes usage for backward compatibility.
• Add maintainer

Related issues

• Close feat: plugin Index #6641

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

@DmitriyLewen I'm still writing a developer guide, but the implementation has been completed. I would appreciate your quick feedback.

[DmitriyLewen]

Left some comments.

Also prefix log is broken:

➜  ./trivy plugin install aqua
2024/05/13 12:40:43 INFO Installing the plugin... prefix="[plugin] " src=github.com/aquasecurity/trivy-plugin-aqua

[DmitriyLewen]

1 more thought:
perhaps we want to add information that the plugin was installed successfully?

Currently:

➜  trivy git:(feat/plugin_index) ✗ ./trivy plugin install kubectl
2024/05/13 12:57:56 INFO Installing the plugin... prefix="[plugin] " src=github.com/aquasecurity/trivy-plugin-kubectl

[knqyf263]

Also prefix log is broken:

Thanks. It worked at the beginning, and got broken after refactoring. Fixed in 9dbecdf

[knqyf263]

perhaps we want to add information that the plugin was installed successfully?

Done
466b9f5

[knqyf263]

@DmitriyLewen I've addressed your comments and completed the developer guide. This PR is now ready for review.

[DmitriyLewen]

LGTM