[elm] Add possibility to override basePath

Created by: curcas

Description

Generating the client for elm takes the current url as the basePath and gives no possibility to override it.

This is absolutely OK when an external API is used. But generating the client for a project which is shipped together with the API has always localhost in it. This should be customizable with the current URL under which the project is served.

openapi-generator version

v3.3.1

OpenAPI declaration file content or url

Not a specific one.

Command line used for generation

generate -i http://localhost/swagger/v1/swagger.json -g elm -o /generated

Steps to reproduce

1. Have a swagger.json file
2. Generate the elm client with the command above
3. Open one of the Request files
4. See the basePath is localhost and cannot be overriden or passed in.

Related issues/PRs

Suggest a fix/enhancement