feat: OpenApi schema

[davidgamez]

Description

Changes:

• Add OpenAPI specification file and Swagger UI.
• Remove the functions that return dynamically the OpenAPI.
• Add CORS support to feeds and validator functions.

Swagger UI is accessible at https://mobilitydata.github.io/gbfs-validator/SwaggerUI/index.html

Testing

For testing the SwaggerUI use the third server option: https://deploy-preview-171--gbfs-validator.netlify.app/.netlify/functions.

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