auth: get credentials from .env (dotenv) in project root

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

As I work across multiple organisations, I normally store AWS credentials in a .env file in the root project directory, to prevent inadvertently deploying something to the wrong organisation. This .env file is not checked in to source control.

**Describe the solution you'd like**

1. Load a dotenv file if it exists in the project root
1. Detect credentials in environment variables.
1. Allow configuring the .dotenv path (I don't need this myself, but I imagine others might like it.)

**Describe alternatives you've considered**

1. Switch to using profiles, without a default profile. Would still require the use of environment variables to select the profile, but the profile could (I believe) be selected manually in vscode. Doesn't entirely give equivalent functionality. An advantage over the current setup would be enabling the use of temporary credentials. This would have some advantages, but would also be a little bit less ergonomic than the current approach.

The issue names no files, tests, or entry points. Start by locating the existing AWS credential resolution flow, then determine how dotenv loading, environment-variable detection, and a configurable dotenv path should fit together; done means the requested credential sources work without checking the .env file into source control.

Written by the indexing model from the issue text.