Generate plugin index file

[bentsherman]

This PR adds a buildIndex task which generates a plugin index file using a custom entrypoint in the Nextflow runtime

It is a Java task that loads the Nextflow runtime (requires 25.08.0-edge or later) and the plugin that was just built, so that it can load the plugin extension points and generate the index file using the PluginIndexWriter class in the Nextflow runtime.

Depends on nextflow-io/nextflow#6361

TODO:

• include the index JSON when publishing the plugin to the registry
• test with the registry

[pditommaso]

@bentsherman Let's aim moving this to "Ready"

[bentsherman]

I am testing the entire flow with the registry locally. Before we merge this PR, we need to:

1. Finalize and merge https://github.com/seqeralabs/plugin-registry/issues/171
2. Finalize and merge Generate plugin index file nextflow#6361 (I need to review the schema structure)
3. Release Nextflow 25.08.0-edge

[pditommaso]

You should be able to test everything in the local dev. Check this for using unreleased plugin https://github.com/nextflow-io/nextflow/blob/a5c19b895a5d64ef4ae76907b5f38dc7074df22e/settings.gradle#L17-L19