Cannot use ignore_not_sysadmin validator

I'm trying to create an organization field that is only editable by sysadmins. I don't care if org admins can see it, but I don't want them to be able to change it (or clear it). Here's my attempt at such a field:
```
{
"field_name": "request_limit",
"label": "Request Limit",
"form_placeholder": "0",
"validators": "ignore_not_sysadmin"
}
```

But when I try to edit the organization as a non-sysadmin, I get this exception (I guess because the `ignore_not_sysadmin` validator deletes the key):
```
Traceback (most recent call last):
File "/usr/lib/python3.8/site-packages/flask/app.py", line 1949, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/lib/python3.8/site-packages/flask/app.py", line 1935, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/usr/lib/python3.8/site-packages/flask/views.py", line 89, in view
return self.dispatch_request(*args, **kwargs)
File "/usr/lib/python3.8/site-packages/flask/views.py", line 163, in dispatch_request
return meth(*args, **kwargs)
File "/srv/app/src/ckan/ckan/config/middleware/../../views/group.py", line 955, in post
group = _action(u'group_update')(context, data_dict)
File "/srv/app/src/ckan/ckan/logic/__init__.py", line 504, in wrapped
result = _action(context, data_dict, **kw)
File "/srv/app/src/ckan/ckan/logic/action/update.py", line 812, in organization_update
return _group_or_org_update(context, data_dict, is_org=True)
File "/srv/app/src/ckan/ckan/logic/action/update.py", line 684, in _group_or_org_update
data, errors = lib_plugins.plugin_validate(
File "/srv/app/src/ckan/ckan/lib/plugins.py", line 312, in plugin_validate
result = plugin.validate(context, data_dict, schema, action)
File "/srv/app/src/ckanext-scheming/ckanext/scheming/plugins.py", line 191, in validate
return navl_validate(data_dict, schema, context)
File "/srv/app/src/ckan/ckan/lib/navl/dictization_functions.py", line 285, in validate
converted_data, errors = _validate(flattened, schema, validators_context)
File "/srv/app/src/ckan/ckan/lib/navl/dictization_functions.py", line 335, in _validate
convert(converter, key, converted_data, errors, context)
File "/srv/app/src/ckan/ckan/lib/navl/dictization_functions.py", line 237, in convert
converter(key, converted_data, errors, context)
File "/srv/app/src/ckan/ckan/logic/converters.py", line 24, in convert_to_extras
data[('extras', new_index, 'value')] = data[key]
KeyError: ('forecast_daily_locations_limit',)
```

If I add `ignore_missing` to the validators, no exception is raised, but editing the org as an org admin clears the field.

Any way to accomplish this?

Thanks!

Start in ckanext-scheming/ckanext/scheming/plugins.py and follow the validation path shown through ckan/ckan/lib/navl/dictization_functions.py and ckan/ckan/logic/converters.py. Reproduce the organization edit with ignore_not_sysadmin, compare it with ignore_missing, and verify that an org admin cannot change or clear the existing field without triggering the KeyError.

Written by the indexing model from the issue text.