Openapi.yaml doc for API

[mggevaer]

I created an Openapi doc, this way it's easier to generate SDK's and try out the API.
I don't think there's much more information which can be captured in the OpenAPI doc than what I've got in there now.

Some concerns.

• The enum for supported countries is hard-coded and does not auto-update with the returned value of the supported countries API call.
• Currently assumes all conversion rates have a precision of 0.0001

[mggevaer]

Let me know if anything needs to happen or looks wrong before this could be merged.

[hakanensari]

Hi @mggevaer! Thanks for contributing this.

We actually already have an OpenAPI specification at lib/public/v1/openapi.json, served at https://api.frankfurter.dev/v1/openapi.json.

Your spec has more detailed schemas, but maybe some, as you pointed out, are too rigid or could become outdated.

We could cherry-pick some of your better schema definitions to enhance the existing openapi.json while keeping the flexibility that matches actual API behavior?