support for specs with numeric schema names

[ericphanson]

e.g. https://opendata.aemet.es/AEMET_OpenData_specification.json uses "200" as a schema name, but open-api-generator fails to generate the schemas correctly (and also doesn't throw an error!).

[aj-foster]

Hi there 👋🏼

This is an interesting case. In order to make a valid Elixir module name, we'll need to prefix the schema name with something. Do you have any suggestions what might be a good choice? (ex. S200)