Backend for reference metadata fetching

[juliusknorr]

• Requires Add fusonic/opengraph 3rdparty#1131

Summary

This is about showing a preview of links posted in Talk, like what is done when posting links on other chat platforms.

And this is the backend part, is not specific to Talk, and may not be specific to links in the future, which is what makes the original description convoluted.

This PR implements an API to be used to generate a rich preview rendering of references (which are links in the first iteration). The backend gathers additional metadata for references through different providers, which can be added by apps, but there is a fallback to an open graph provider by default. The open graph extraction can be turned off through config.php.

ToDo

• Frontend integration Implement reference preview rendering nextcloud-libraries/vue-richtext#743

Backend

• Implement a provider based API to fetch metadata for a link reference
  • Fallback to open graph fetching for unknown link references
    • config option to disable opengraph
    • Cache images in app data
      • Limit to sane content types
      • Limit in size
  • Reference metadata is stored in memory cache only as a database might not bring much benefit here
  • Providers can expose how their metadata should be cached (globally or per unique cache key, e.g. the user id)
  • Allow provider registration through Bootstrap
  • rate limit for uncached references
• OCS API endpoints for fetching the metadata from the frontend
• OCP API for fetching the metadata in apps

Integration

• Talk (vue-richtext)
• Text (custom implementation but reusing vue-richtext components)
• Comments (would need to migrate to vue-richtext)

Provider implementations

• Server
  • Files
  • Profile
• Talk conversation
• Deck card

[come-nc]

It took me so long to understand what this is about, so, for other unaware reviewers:
This is about showing a preview of links posted in Talk, like what is done when posting links on other chat platforms.

And this is the backend part, is not specific to Talk, and may not be specific to links in the future, which is what makes the original description convoluted.

[juliusknorr]

Sorry, should have clarified that a bit more, but I've taken your description for the first post in addition

[juliusknorr]

Failures unrelated

[come-nc]

The phpdoc description is wrong

[juliusknorr]

Documentation at https://github.com/nextcloud/documentation/blob/master/developer_manual/digging_deeper/reference.rst#L8