Add vapid_key to the application entity in the REST API (mastodon#10058)

Fix mastodon#8785

Author: Gargron

app/controllers/api/v1/apps/credentials_controller.rb

@@ -6,6 +6,6 @@ class Api::V1::Apps::CredentialsController < Api::BaseController
   respond_to :json
 
   def show
-    render json: doorkeeper_token.application, serializer: REST::StatusSerializer::ApplicationSerializer
+    render json: doorkeeper_token.application, serializer: REST::ApplicationSerializer, fields: %i(name website vapid_key)
   end
 end

app/serializers/rest/application_serializer.rb

@@ -2,7 +2,7 @@
 
 class REST::ApplicationSerializer < ActiveModel::Serializer
   attributes :id, :name, :website, :redirect_uri,
-             :client_id, :client_secret
+             :client_id, :client_secret, :vapid_key
 
   def id
     object.id.to_s
@@ -19,4 +19,8 @@ def client_secret
   def website
     object.website.presence
   end
+
+  def vapid_key
+    Rails.configuration.x.vapid_public_key
+  end
 end