Support a new schema of plugin manifest

[knqyf263]

Description

Trivy added some changes to a schema of plugin.yaml for the plugin index. The changes are as follows:

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

Example: aquasecurity/trivy-plugin-count@b98cf06

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ knqyf263
❌ tzurielweisberg
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[knqyf263]

@naortalmor1 @tamirkiviti13 Could you review this PR?

[tzurielweisberg]

Need to fix https://github.com/aquasecurity/trivy-plugin-aqua/blob/master/.github/plugin_template.yaml#L4 as well

[knqyf263]

@tzurielweisberg Done 76e56ce