feat: Business hour Inbox APIs (#1821)

* feat: Business hour Inbox APIs

app/models/inbox.rb

@@ -12,6 +12,7 @@
 #  greeting_message       :string
 #  name                   :string           not null
 #  out_of_office_message  :string
+#  timezone               :string           default("UTC")
 #  working_hours_enabled  :boolean          default(FALSE)
 #  created_at             :datetime         not null
 #  updated_at             :datetime         not null
@@ -29,6 +30,7 @@ class Inbox < ApplicationRecord
   include OutOfOffisable
 
   validates :account_id, presence: true
+  validates :timezone, inclusion: { in: TZInfo::Timezone.all_identifiers }
 
   belongs_to :account