Add more fields to BackendConfig: #758

Job	Run time
integration (integration test, .ci/tests/integration/e2e.yaml)	22m 32s
integration (integration test with tls, .ci/tests/integration/e2e_with_tls.yaml)	22m 16s
integration (integration test with oauth2, .ci/tests/integration-oauth2/e2e.yaml)	22m 33s
integration (integration test with downloader, .ci/tests/integration-oauth2/e2e_with_downloader.y...	22m 32s
	1h 29m 53s

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