[RFC] Patch building and Webhooks API

Webhook API should help to integrate `KernelCI` with patch management (Patchwork) and version control systems (Github, Gitlab). It will provide interface to trigger non-upstream patch builds that should be able to publish results back later.

## Implementation
As the first step, I'd like to implement Patchwork integration with KernelCI. Following changes should be implemented:
1. Add `/webhooks/patchwork` API that will expect certain (TBD) input from Patchwork side, enough to build and test kernel, and report back results
2. Extend `kernelci.Node`, `KernelBuildMetadata`, `kernelci.config.Tree` with patch-related fields
3. Implement patch mbox application on top of git checkout
4. Implement Patchwork patch checks update mechanism

## Minimal patch information
We need a patch and all dependent patches information, as well as submitter information for email notifications.
```
{
"patches": [
{
"patchwork_id": "str",
"hash": "str",
"web_url": "str",
"date": "str",
"mbox": "str"
}
],
"submitter": {
"id": "int",
"url": "str",
"name": "str",
"email": "str"
}
}
```

## Discussion
- How to make patches information generic enough to share across multiple systems? Should we make it generic at all?
- How to pass Patchwork specific metadata into `Node`? `data` field or a separate field?
- Calculating and passing revision information
- What data should we expect in `/webhooks/patchwork`? (Collaboration with Patchwork developers)

No files, tests, or entry points are named. Start by reading the RFC and its unresolved Discussion questions, then map the four requested implementation areas: the Patchwork webhook, patch metadata, mbox application, and check updates. Done requires an agreed webhook contract and support for building, testing, and reporting patch results.

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