inject AWS credentials to any launch config

## Problem

Customers may want to run arbitrary tests with the current AWS credentials passed to the tests (which presumably use the AWS SDKs). Example: https://github.com/aws/aws-toolkit-jetbrains/issues/3417

## Expected behavior

- From https://github.com/aws/aws-toolkit-vscode/issues/3673:
> Using any AWS SDK the default credential provider chain should find the "injected" Toolkit credentials.
- Toolkit provides a `Set AWS Credentials in Child Processes` feature which passes the current credentials to vscode launch configs when they are run/debugged.

## Implementation note

- Possible approach: set the `AWS_X` environment variables of the vscode process.
- Problem: this will also affect `sam` invocations, etc. Could be confusing if one forgets that this is enabled.
- Possible approach: does vscode allow extensions to provide custom fields on arbitrary launch configs?
- Then one could add `${aws_credentials}` or `"aws_credentials": true` to a launch config, and AWS Toolkit could inject `AWS_X` env vars only for that launch-config.

No repository files or tests are named. Start by reading the linked VS Code issue 3673 and investigating how VS Code extensions can add fields to arbitrary launch configurations, alongside the existing “Set AWS Credentials in Child Processes” feature. Done means an opted-in launch configuration passes Toolkit credentials so AWS SDK default provider chains find them without unexpectedly affecting other invocations such as sam.

Written by the indexing model from the issue text.