Feature Request: (Reply) Context import (converted to project)

This issue has been converted to a project [Feature Request: (Reply) Context import](https://linear.app/a8c/project/feature-request-reply-context-import-0e388252ce73)

---

### What

Parse the `reply`-collection and `context` of an incoming reply/post.

### Why

Fill missing comments in a comment thread or add all comments of a post that where made before a post was indexed.

### How

See:

* [FEP-7888: Demystifying the context property](https://codeberg.org/fediverse/fep/src/branch/main/fep/7888/fep-7888.md)
* [FEP-2931: Representing context with a Collection](https://codeberg.org/fediverse/fep/src/branch/main/fep/2931/fep-2931.md)
* [FEP-11dd: Context Ownership and Inheritance](https://codeberg.org/fediverse/fep/src/branch/main/fep/11dd/fep-11dd.md)
* [Replies (Mastodon Doc)](https://docs.joinmastodon.org/spec/activitypub/#:\~:text=replies)