feat: add 'generate' subcommand to create a config skeleton

Run `deck file generate` to generate a config file skeleton as initially requested [here](https://github.com/Kong/deck/issues/653#issuecomment-1106736676)

## Acceptance criteria:

Generate a sample file that contains the following entities:

- [ ] Service
- [ ] Route (attached to service)
- [ ] Consumer
- [ ] Rate Limiting plugin (attached to service and consumer)

It should generate a configuration with each entity at the top level, using foreign keys for references

Start by tracing the `deck file` command and its configuration serialization or generation entry points. Determine how a generated skeleton can represent Service, Route, Consumer, and Rate Limiting plugin entities at the top level with foreign-key references. Done means `deck file generate` emits a sample configuration containing all four entities and the specified attachments.

Written by the indexing model from the issue text.