Enhanced shared configuration

**Is your feature request related to a problem? Please describe.**

Following up from https://github.com/intuit/auto/issues/191, add support for something like [preset hosting](https://docs.renovatebot.com/config-presets/#preset-hosting) and [local presets](https://docs.renovatebot.com/config-presets/#local-presets) (for self-hosted ghes), which I use with [renovate](https://github.com/renovatebot/renovate).

**Describe the solution you'd like**

I'd like to be able to share auto config by checking in a config per repo that simply points to this shared config (and allows extending it). For example, with renovate I can have the following config in a ghes repo, which causes renovate to load the config directly from ghes:

```json
{
"$schema": "https://docs.renovatebot.com/renovate-schema.json",
"extends": [
"config:base",
"local>owner/repo:renovate-default"
]
}
```

The config can also be customized further by adding repo specific sections.

**Describe alternatives you've considered**

* Copy the same config across all repos
* Drifts from ideally shared config over time
* Download the config before running auto in a GitHub Action
* Must be downloaded to repo root (runner working directory) and added to `.gitignore`; messy to customize

**Additional context**