Add more fields to BackendConfig: #762

Job	Run time
integration (integration test, .ci/tests/integration/e2e.yaml)	3m 42s
integration (integration test with tls, .ci/tests/integration/e2e_with_tls.yaml)	3m 29s
integration (integration test with oauth2, .ci/tests/integration-oauth2/e2e.yaml)	3m 41s
integration (integration test with downloader, .ci/tests/integration-oauth2/e2e_with_downloader.y...	3m 40s
	14m 32s

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