Introduce an "init" subcommand for bootstrapping a package

Introduce an "init" subcommand for bootstrapping a package.

At the very least, should bootstrap (decision: client vs server, asked by the CLI):
- Package.swift
- a new Hello World-like OpenAPI document (or an existing one can be provided by the user in the command)
- main.swift/Tool.swift with the entry point and basic code to get things working with `swift run`
- config file with reasonable defaults

For example: `swift-openapi-generator init-package --mode client` would create a new package, with a simple OpenAPI doc, and the generator configured etc. This package would work out of the box with `swift run`. Just need two variants: client and server.

Start by tracing the existing CLI subcommand handling and package configuration, then compare the requested client and server outputs. Done means an init-package command creates Package.swift, a Hello World-like OpenAPI document, main.swift or Tool.swift, and a config file, with the generated package working under swift run.

Written by the indexing model from the issue text.