Extend the openapi model with helper/builder classes/methods

[adamw]

The goal is to create an API for working with the OpenAPI model so that it would be feasible to define a single PathItem (endpoint) "by hand". While the goal of tapir is for most of the endpoints to be defined as tapir.Endpoint, there will be for sure cases where a manually-defined endpoint will be necessary. The openAPI API will allow creating documentation for these endpoints in a humane way.