refactor(core): Move backend config to a separate package

[netroy]

This PR moves the config related code into a separate @n8n/config package so that we can use the config on all backend packages (like core and nodes-base) without creating a circular dependency with cli.
This PR also changes the config object to be

1. an injectable class, that can be used with regular DI code
2. type-safe without the need for the complicated inference code in packages/cli/node_modules/@types/convict/index.d.ts
3. annotated with JSDocs on every config property, which adds helpful tooltips in our editors
   

Every sub-section in the config is also now separately injectable, which we can use to create config objects specific to certain sections of the codebase, instead of using this one global config everywhere.

TODO

• Move the rest of the config properties into the new package
• Read values from files for env variables with _FILE suffix
• Generate a JSON file when building @n8n/config, include it in the published package, and then use this JSON to generate the docs for env variables.
• Merge InstanceSettings into this package
• Support reading all properties from .n8n/config

Review / Merge checklist

• PR title and summary are descriptive
• Docs updated or follow-up ticket created.
• Tests included

[ivov]

Ideally there should be a way to have the effect of readonly but without overly narrowing the type and without having to specify it... Not sure if possible but I'll look if there's a way.