Add support for calling worker plugins from URL set in the manifest

[rndquu]

Depends on ubiquity-os/plugin-template#33

This task introduces a new property homepage_url (which is a plugin worker URL) in the manifest.json file.

The idea is that now the bot's config will contain only org/repo in the plugin's config. For example here instead of

- plugin: https://ubiquity-os-comment-vector-embeddings-development.ubiquity.workers.dev

we will set org/repo value:

- plugin: ubiquity-os-marketplace/text-vector-embeddings

The kernel should support calling a worker URL if it is set in the manifest.json.

How it is supposed to work:

1. If plugin config property contains URL then call it as a worker (this is to support close sourced plugins + for backwards compatibility)
2. If plugin config property contains org/repo value then fetch plugin's manifest.json:

• If homepage_url property is present in the manifest then call the plugin as a worker
• If homepage_url property is missing then call plugin as a github action

Original comment