Generate plugin index file

[bentsherman]

This PR adds an entrypoint for generating a "plugin index file", which contains a list of plugin definitions (config scopes, functions, etc).

It should be used by the Nextflow Gradle plugin to generate the index file when building/releasing a plugin, and it should be consumed by the plugin registry for use by the language server, registry UI, etc.

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[pditommaso]

Does not look to have dependency with the nextflow runtime, does it? could make sense to keep it in its own module or even keep it in the gradle plugin source? it would be preferable to have the gradle plugin to no depend on the main nextflow runtime

[bentsherman]

The PluginIndexWriter does depend pretty heavily on runtime classes, mainly around plugin extensions and config schema. This is why I placed it in a separate configuration, so that the main build doesn't depend on the specific Nextflow version

Long-term, I think we'll want to have a module in Nextflow that defines a versioned "runtime API" that is stable across Nextflow versions. For now I'm just trying to make it work