Missing fields in json formatter

[sixela]

Describe the bug As of now some available fields can't be displayed when using --json flag, e.g "attendees"

To Reproduce

khal list --json attendees today today
[{}, {}] # List will vary depending on your calendars

Expected behavior

I should obtain a json-formatted list of list of attendees like:

khal list --json attendees today today
[{"attendees": ""}, {"attendees": "attendee1, attendee2"}]

OS, version, khal version and how you installed it:

• The output of khal --version: khal, version 0.11.3.dev160+g214886d
• Installation method [e.g. PyPI, git, OS repo]: pip install git+https://github.com/pimutils/khal
• python version: 3.12
• OS: Darwin Kernel Version 22.5.0
• Your khal config file

  
  [default]
  default_calendar = <redacted>
  highlight_event_days = True

[view] event_view_always_visible = True

 - The versions of your other python packages 

$ pip freeze pip freeze atomicwrites==1.4.1 click==8.1.7 click-log==0.4.0 configobj==5.0.8 icalendar==5.0.10 khal @ git+https://github.com/pimutils/khal@214886de48bf706f897592cd01a9de6250ba3df4 python-dateutil==2.8.2 pytz==2023.3.post1 pyxdg==0.28 six==1.16.0 tzlocal==5.2 urwid==2.2.3

**Additional context**
The additionnal attributes are available but are ignored because of the CONTENT_ATTRIBUTES var at https://github.com/pimutils/khal/blob/master/khal/utils.py#L218 

For my needs I manually patched like so:

diff --git a/khal/utils.py b/khal/utils.py index 78f525b..41d40c9 100644 --- a/khal/utils.py +++ b/khal/utils.py @@ -223,7 +223,7 @@ CONTENT_ATTRIBUTES = ['start', 'start-long', 'start-date', 'start-date-long', 'start-style', 'end-style', 'to-style', 'start-end-time-style', 'end-necessary', 'end-necessary-long', 'repeat-symbol', 'repeat-pattern', 'title', 'organizer', 'description', 'location', 'all-day', 'categories',

• 'uid', 'url', 'calendar', 'calendar-color', 'status', 'cancelled']

• 'uid', 'url', 'calendar', 'calendar-color', 'status', 'cancelled', 'attendees']

  def json_formatter(fields)