feat: file conversion provider

[elzody]

• Part of: 📄💫 File conversion server#49603
• Target version: main

Summary

To test file conversion, send a network request like the following:

curl --request POST \
  --url 'http://nextcloud.local/ocs/v2.php/apps/files/api/v1/convert?format=json' \
  --header 'Authorization: Basic YWRtaW46YWRtaW4=' \
  --header 'Content-Type: application/json' \
  --header 'ocs-apirequest: true' \
  --data '{
  "fileId": 1337,
  "targetMimeType": "target/mime+type"
}'

TODO

Testing will probably be in a follow-up PR.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required

[juliusknorr]

Looks good, one additional comment regarding formats.

[elzody]

/backport to stable31

[blizzz]

skimmed it, did not spot anything weird (but also did not spot tests 🤐)