feat(conversations): add opt-in merge for custom attributes (#15119)

Adds an opt-in merge flag to the conversation custom attributes endpoint
so integrations can update only the keys they send instead of replacing
the whole hash, matching how the contacts endpoint already behaves. Also
adds a destroy_custom_attributes endpoint to remove specific keys,
mirroring the contacts convention. Replace stays the default, so
existing integrations are unaffected.

How to test

POST /conversations/:id/custom_attributes com { "custom_attributes":
{"a":1} }, then { "custom_attributes": {"b":2}, "merge": true } results
in {a:1, b:2}; no merge, results in {b:2}.
POST /conversations/:id/destroy_custom_attributes with {
"custom_attributes": ["a"] } removes only a.

---------

Co-authored-by: Vishnu Narayanan <iamwishnu@gmail.com>

config/routes.rb

@@ -173,6 +173,7 @@ Rails.application.routes.draw do
               post :update_last_seen
               post :unread
               post :custom_attributes
+              post :destroy_custom_attributes
               get :attachments
               get :inbox_assistant
               get :reporting_events if ChatwootApp.enterprise?