feat(openapi): Allow setting info extensions in OApi specifications

[amtelekom]

The OpenAPI Info object may be extended with additional fields, e.g. from the OApi 3.0.3 Spec:

4.7.2 Info Object

[...]
This object MAY be extended with Specification Extensions

This PR allows setting such extensions via poem-openapi:

use serde_json::Value as JsonVal;

OpenApiService::new(...)
    // other settings
    .info_extension("x-category", JsonVal::String("Example".to_string()))
    .info_extension("x-segment", JsonVal::Array(vec![
        JsonVal::String("B2B".to_string()),
        JsonVal::String("B2C".to_string())]));

results in the extensions being added in the JSON and YAML spec endpoints:

{
  "openapi": "3.0.0",
  "info": {
    // ...
    "x-category": "Example",
    "x-segment": [
      "B2B",
      "B2C"
    ]
  }
  // ...
}

openapi: 3.0.0
info:
  #...
  x-category: Example
  x-segment:
  - B2B
  - B2C
# ...

I guess this implements at least a small part #908

[attila-lin]

would you mind add some test case? thank you!

[amtelekom]

Hi,

sorry, I was busy the last few days, but I've now added a test case that checks the extensions are correctly encoded into the spec

[attila-lin]

LGTM
Thank you.