use urls from spec

[ethanniser]

https://swagger.io/docs/specification/v3_0/api-host-and-base-path/

maybe a way to make it easier to surface baseurls for the client from this part of the spec

[tim-smart]

Could try inject something here: https://github.com/tim-smart/openapi-gen/blob/main/src/OpenApi.ts#L322