OpenAPI IP address support

[FallingSnow]

OpenAPI IP addresses should be supported as a format (in poem-openapi as a type).

OpenAPI does not make reference to them in the specification but they seem to be supported.
See swagger-api/swagger-ui#5033 for support on this point.

I recommend adding type support for Ipv4Addr as it is an existing struct.

[sunli829]

It's easy, do it like below, would you like to open a PR to help me with this? 🙂

https://github.com/poem-web/poem/blob/master/poem-openapi/src/types/external/uuid.rs

[FallingSnow]

I tried it, mostly just replaced anywhere it says uuid. Not sure if I did it right.