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>

spec/factories/integrations/hooks.rb

@@ -29,7 +29,7 @@ FactoryBot.define do
 
     trait :linear do
       app_id { 'linear' }
-      settings { { api_key: 'api_key' } }
+      access_token { SecureRandom.hex }
     end
   end
 end