custom_profile_fields: Add "editable_by_user" setting.

This new property allows organization administrators to specify whether
users can modify the custom profile field value on their own account.
This property is configurable for individual fields.

By default, existing and newly created fields have this property set to
true, that is, they allow users to edit the value of the fields.

Fixes part of #22883.

Co-Authored-By: Ujjawal Modi <umodi2003@gmail.com>

zerver/lib/types.py

@@ -25,6 +25,7 @@ class ProfileDataElementBase(TypedDict, total=False):
     hint: str
     display_in_profile_summary: bool
     required: bool
+    editable_by_user: bool
     field_data: str
     order: int