BUG/MINOR: add missing field when parsing backend `balance` field #554

Job	Run time
generate-models	1m 7s
HAProxy check commit message	3s
generate-specification	47s
lint-spec	9s
Build	29s
license-check	19s
golangci-lint	59s
go mod tidy	6s
Tests	3m 13s
e2e on HAProxy (2.1)	44s
e2e on HAProxy (2.2)	44s
e2e on HAProxy (2.3)	44s
e2e on HAProxy (2.4)	41s
e2e on HAProxy (2.5)	44s
e2e on HAProxy (2.6)	42s
e2e on HAProxy (2.7)	46s
e2e on HAProxy (2.8)	41s
	12m 58s

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