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>

lib/integrations/linear/processor_service.rb

@@ -76,7 +76,6 @@ class Integrations::Linear::ProcessorService
   end
 
   def linear_client
-    credentials = linear_hook.settings
-    @linear_client ||= Linear.new(credentials['api_key'])
+    @linear_client ||= Linear.new(linear_hook.access_token)
   end
 end