MEDIUM: switch models generation to go-method-gen #1228

Job	Run time
generate-models	1m 25s
generate-specification	15s
HAProxy check commit message	-1s
golangci-lint	0s
Build	0s
license-check	0s
lint-spec	0s
Test equal	0s
Tests	0s
go mod tidy	0s
e2e on HAProxy	-1s
	1m 38s

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