Support secrets in Lambda environment

[twelch]

geoprocessing functions need to be able to access outside resources using API keys or other secrets.

There is a .env file that the developer sets up when running the devcontainer environment. But that only makes .env variables available when running tests locally.

We don't want to just insert all the .env file secrets into the Lambda environment on build/deploy. The framework needs a mechanism for the developer to specify which credentials should be made available to the Lambda environment.

[twelch]

Suggest using a prefix. any environment variable starting with GP_LAMBDA gets forwarded to the Lambda environment