Add extra-apt-sources in API

• added a migration file to add field extra_apt_sources to the jobs table.
• updated docs of POST /test/:suite/:arch in api.rb to show extra-apt-sources as part of tests object and docs of GET /test/ to show extra_apt_sources as a new key in respnse JSON.
• updated request_tests functionintest_handler.rbto addextra_apt_sources` as part of new job objects.
• updated enqueue_parameters method in job.rb to accept extra-apt-sources as a new parameter which is iterated and encoded to base64.
• updated debci-worker to accept extra-apt-sources and send to debci-test.
• updated debci-test to decode extra-apt-sources from base64 and pass as add-apt-source to autopkgtest.
• added tests for same.