Support `merge` in `OpenApi` derive #2988

Job	Run time
test (examples)	8m 41s
typos	3s
test (utoipa)	4s
test (utoipa-gen)	2m 31s
test (utoipa-swagger-ui-vendored)	6s
test (utoipa-swagger-ui)	5s
test (utoipa-redoc)	10s
test (utoipa-rapidoc)	5s
test (utoipa-scalar)	4s
test (utoipa-axum)	8s
test (utoipa-config)	5s
test (utoipa-actix-web)	5s
	12m 7s

Provide feedback