Add support for output format to `occ files:scan`

Steps to reproduce

1.occ files:scan --output=json_pretty -- user

{"reqId":"UPIugY3Zgb2Oo5c0EFiw","level":1,"time":"2021-05-19T10:20:31+00:00","remoteAddr":"","user":"--","app":"admin_audit","method":"","url":"--","message":"Console command executed: files:scan --output=json_pretty -- user","userAgent":"--","version":"19.0.8.1"}
Unknown user 1 user
+---------+-------+--------------+
| Folders | Files | Elapsed time |
+---------+-------+--------------+
| 0       | 0     | 00:00:00     |
+---------+-------+--------------+

occ files:scan --output=json -- user

{"reqId":"71yPn7HwMRty0OuhXFXH","level":1,"time":"2021-05-19T10:20:40+00:00","remoteAddr":"","user":"--","app":"admin_audit","method":"","url":"--","message":"Console command executed: files:scan --output=json -- user","userAgent":"--","version":"19.0.8.1"}
Unknown user 1 user
+---------+-------+--------------+
| Folders | Files | Elapsed time |
+---------+-------+--------------+
| 0       | 0     | 00:00:00     |
+---------+-------+--------------+

occ files:scan --output=plain -- user

{"reqId":"7SR7CkmnEtTrcZXtfYFX","level":1,"time":"2021-05-19T10:20:46+00:00","remoteAddr":"","user":"--","app":"admin_audit","method":"","url":"--","message":"Console command executed: files:scan --output=plain -- user","userAgent":"--","version":"19.0.8.1"}
Unknown user 1 user
+---------+-------+--------------+
| Folders | Files | Elapsed time |
+---------+-------+--------------+
| 0       | 0     | 00:00:00     |
+---------+-------+--------------+

Expected behaviour

The output format should actually change. In conjunction with #27029 this makes parsing for errors even harder.

Server configuration

Nextcloud version:
19.0.8.1

Start by locating the implementation of the occ files:scan command and reproduce the three --output examples from the issue. Trace how the requested output format is parsed and rendered, then verify that each format changes the command output and that errors remain distinguishable.