feat: Linear OAuth 2.0 (#10851)

Fixes https://linear.app/chatwoot/issue/CW-3417/oauth-20-authentication
We are planning to publish the Chatwoot app in the Linear [integration
list](https://linear.app/docs/integration-directory). While we currently
use token-based authentication, Linear recommends OAuth2 authentication.
This PR implements OAuth2 support.

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

config/routes.rb

@@ -233,6 +233,7 @@ Rails.application.routes.draw do
             end
             resource :linear, controller: 'linear', only: [] do
               collection do
+                delete :destroy
                 get :teams
                 get :team_entities
                 post :create_issue
@@ -444,6 +445,10 @@ Rails.application.routes.draw do
     resource :callback, only: [:show]
   end
 
+  namespace :linear do
+    resource :callback, only: [:show]
+  end
+
   namespace :twilio do
     resources :callback, only: [:create]
     resources :delivery_status, only: [:create]