Fix AWS Lambda/Gateway API stage issue

[jamesfisher-geo]

Is your feature request related to a problem? Please describe.

When deploying sfeos as an AWS Serverless Lambda Function with an API Gateway I hit an issue with the root URL. Gateway API requires one to deploy an API to a stage. The name of which is appended to the root URL of the API (https://{api_Id}.execute-api.{region}.amazonaws.com/{stage}/, for example). However, the Starlette base_url parameter in the request does not include the stage name. This results in all generated links yielding a 403 error (https://{api_Id}.execute-api.{region}.amazonaws.com/collections/ rather than https://{api_Id}.execute-api.{region}.amazonaws.com/{stage}/collections/).

Describe the solution you'd like
I found this issue which found a solution by adding app.root_path = '/{stage} to app.py. I propose we add an environment variable to enable devs to set the stage name as a workaround for this Gateway API issue.

Describe alternatives you've considered
Others have solved this by using Forwarded or x-Forwarded` proxy headers. However, the Gateway API does did not let me implement proxy headers when connecting to a Lambda function.