feat: Support Regex validation for custom attributes (#7856)

This allows a user to add/update a custom regex and a cue while defining custom attributes(Only applicable for type- text).
While adding/editing custom attributes, the values are validated against the attribute definition regex, and if it is incorrect, a cue message or default error message is shown and restricts invalid values from being saved.

Fixes: #6866

app/models/channel/web_widget.rb

@@ -35,7 +35,7 @@ class Channel::WebWidget < ApplicationRecord
                     { pre_chat_form_options: [:pre_chat_message, :require_email,
                                               { pre_chat_fields:
                                                 [:field_type, :label, :placeholder, :name, :enabled, :type, :enabled, :required,
-                                                 :locale, { values: [] }] }] },
+                                                 :locale, { values: [] }, :regex_pattern, :regex_cue] }] },
                     { selected_feature_flags: [] }].freeze
 
   before_validation :validate_pre_chat_options