Implement /preview_url

*This issue was originally created by [**@r3k2**](https://github.com/r3k2) at .*

### Background information

- **Dendrite version or git SHA**:
Your branch is up to date with 'origin/main'.
git sha: 6c95f9b535191bc28cb983a6fa0eab036dc40916

- **Monolith or Polylith?**:
Monolith

- **SQLite3 or Postgres?**:
postgresql

- **Running in Docker?**:
Hell no.
- **`go version`**:

go version go1.18 linux/arm64
- **Client used (if applicable)**:
Element-web
Element-Desktop
Gomuks(but this is not related since is console base but it does supposed to render in ASCII)
### Description

After almost 2 years running dendrite I never really got the rss/link/images previews no matter how many times I enable it on element-web or element-desktop etc since this is not a big deal and have other bigger issues to fix I never brought it up.
Who is affected all my users on our dendrite server.

I do not see any logs that will output or spit anything related is just ignored.
I enabled and disable a couple times the render images option to see if it help, today I did again with no luck.

### Steps to reproduce

Someone post a URL and on matrix.org I can see the preview but on my dendrite user account I can not.
Same with bots and anything/anyone that pastes a url or anything to preview.

I am in contact with Keg over matrix but for bigger issues, he has access to my server if needed for troubleshooting.

Start by locating the /preview_url endpoint or URL-preview handling in Dendrite and compare its behavior with the previews observed on matrix.org. Check how Element-web, Element-Desktop, and Gomuks request or render previews, then trace why the reported server produces no logs or previews. Done means posted URLs produce previews for the affected users and the behavior is covered by an appropriate test.

Written by the indexing model from the issue text.