Allow to set customizable codegen URLs at build time

Hi there,

With the switch to React and the release of swagger-editor v5, we lost the ability to configure some codegen URLs.

For instance, for users that had their own Swagger generator server, it was previously possible to set URL_SWAGGER2_GENERATOR, URL_OAS3_GENERATOR or even URL_SWAGGER2_CONVERTER - see https://github.com/swagger-api/swagger-editor/pull/2910/files

Since we cannot do this at runtime now (due to the limitations mentioned in the README), I think it would still be nice to allow users to configure this at build time.

E.g., following the former syntax that we had. In order to disable the OpenAPI 2.x to OpenAPI 3.x convert feature, we would do:

REACT_APP_SWAGGER2_CONVERTER_URL=null npm run build:app

Would the maintainers of this project be open to some PR(s) that would go that route?

/cc @char0n since you initially reviewed https://github.com/swagger-api/swagger-editor/pull/2910

Start with the build:app command and the README section describing environment-variable limitations, then compare the former configuration in pull request 2910. The work is complete when build-time variables can configure the listed codegen and converter URLs, including disabling the converter with the shown null value.