How can you define custom environment-specific variables?

** Which Category is your question related to? **
AWS Amplify cli general

** What AWS Services are you utilizing? **
N/A

** Provide additional details e.g. code snippets **
Is there a way to provide additional environment-specific variables that would be updated (e.g. in `aws-exports.js`) when using `amplify env checkout`?

For example, we manually configure an OAuth domain for each of our environments. We would want to have this as an environment-specific variable which would be available from `aws-exports.js` (or some other place) which would automatically update when we do `amplify env checkout`. (Currently `aws-exports.js` doesn't even contain the environment name, so we cannot create a manual environment map based on it.)

Start by tracing how `amplify env checkout` updates `aws-exports.js` and where environment configuration is generated. Define how user-provided environment-specific values should be stored and exposed, then verify that checking out each environment produces the corresponding values without breaking existing exports.

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