feat: support BackendRef HTTP filters · envoyproxy/gateway@e984aa4

Run make -k lint: internal/gatewayapi/testdata/httproute-with-backendref-add-multiple-filters.in.yaml#L46

46:11 [indentation] wrong indentation: expected 12 but found 10

Run make -k lint: internal/xds/translator/testdata/in/xds-ir/http-route-weighted-backend-with-filters.yaml#L38

38:11 [indentation] wrong indentation: expected 12 but found 10

Run make -k lint: internal/xds/translator/testdata/in/xds-ir/http-route-weighted-backend-with-filters.yaml#L40

40:13 [indentation] wrong indentation: expected 14 but found 12

Run make -k lint: internal/xds/translator/testdata/in/xds-ir/http-route-weighted-backend-with-filters.yaml#L44

44:15 [indentation] wrong indentation: expected 16 but found 14

Run make -k lint: internal/xds/translator/testdata/in/xds-ir/http-route-weighted-backend-with-filters.yaml#L54

54:13 [indentation] wrong indentation: expected 14 but found 12