Make extended plugins optional

[cwperks]

Description

When a plugin extends another extensible plugin, OpenSearch requires the other plugin to be installed ahead of time.

This PR lifts the restriction and allows plugin installation. Instead, this PR warns the cluster administrator that the dependent plugin is missing and then prompts the user to continue with installation. If the dependent plugin is not installed, then the plugin being installed may only offer partial functionality.

While this doesn't make sense in all use-cases, the Security plugin would like to offer a SPI for plugins to share resources. In order for the security plugin to offer a SPI, other plugins would need to declare that they extend the security plugin. The problem with this is that security is optional and plugins currently support clusters where the security plugin is installed or clusters where it is not installed. By making extendedPlugins optional, this will allow security to offer a SPI but not be required for installation.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.