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>

app/models/integrations/app.rb

@@ -1,4 +1,5 @@
 class Integrations::App
+  include Linear::IntegrationHelper
   attr_accessor :params
 
   def initialize(params)
@@ -25,10 +26,18 @@ class Integrations::App
     params[:fields]
   end
 
+  # There is no way to get the account_id from the linear callback
+  # so we are using the generate_linear_token method to generate a token and encode it in the state parameter
+  def encode_state
+    generate_linear_token(Current.account.id)
+  end
+
   def action
     case params[:id]
     when 'slack'
       "#{params[:action]}&client_id=#{ENV.fetch('SLACK_CLIENT_ID', nil)}&redirect_uri=#{self.class.slack_integration_url}"
+    when 'linear'
+      build_linear_action
     else
       params[:action]
     end
@@ -45,6 +54,17 @@ class Integrations::App
     end
   end
 
+  def build_linear_action
+    [
+      "#{params[:action]}?response_type=code",
+      "client_id=#{ENV.fetch('LINEAR_CLIENT_ID', nil)}",
+      "redirect_uri=#{self.class.linear_integration_url}",
+      "state=#{encode_state}",
+      'scope=read,write',
+      'prompt=consent'
+    ].join('&')
+  end
+
   def enabled?(account)
     case params[:id]
     when 'webhook'
@@ -64,6 +84,10 @@ class Integrations::App
     "#{ENV.fetch('FRONTEND_URL', nil)}/app/accounts/#{Current.account.id}/settings/integrations/slack"
   end
 
+  def self.linear_integration_url
+    "#{ENV.fetch('FRONTEND_URL', nil)}/linear/callback"
+  end
+
   class << self
     def apps
       Hashie::Mash.new(APPS_CONFIG)