Add LocalStack endpoint to exports.json

Currently the usage of this utility makes you add `endpoint: https://localhost.localstack.cloud:4566` after importing the exports file. A config that could be easily added to the file using the plugin.

Start by locating how the plugin generates or updates exports.json, then inspect the exported configuration described in the issue. The change is complete when the generated file includes the LocalStack endpoint automatically, so users no longer need to add it after importing the file.

Written by the indexing model from the issue text.