Toolkit does not explicitly set a region for SAM CLI invokes

[bryceitoc9]

Describe the bug
SAM invoke calls (via "Run Locally" and "Debug Locally") will fail if a user does not have a [default] entry in their ~/.aws/config file containing a region value.

To Reproduce

1. Delete/rename/move your ~/.aws/config file
2. Attempt a local run/debug of a Lambda function (all runtimes affected)

Expected behavior
Function runs with a user-defined region tied to the SAM environment (so the AWS_REGION variable will work)

Desktop (please complete the following information):

• OS: All
• Visual Studio Code Version: All
• AWS Toolkit for Visual Studio Code Version: All

Additional context
We have a few implementation options:

• Set the AWS_REGION env var with a valid region code
• Pass the --region flag to the SAM CLI invoke call with a valid region code.
• Pass the --profile flag to the SAM CLI invoke call with the selected AWS profile name. This requires that an ~/.aws/config is created at some point (we don't do this) that has a [profile <profileName>] entry containing a region value

JetBrains sets the AWS_REGION but uses the selected AWS region for the workspace (since they don't have a concept of multiple regions in the explorer).

Aside from pulling the value from ~/.aws/config, we can:

• Pull the first region from the AWS explorer
  • Have the AWS explorer only list one region at a time? That way, we always pull the region in question.
  • Pulling the first region is blocked due to Selected regions in AWS Explorer pane do not persist order after restarting IDE #421
• Add region value to our template.json file
• Prompt the user on run/debug for a region
  • We can cache this value for later
• Use a hardcoded value (this breaks users who are testing the AWS_REGION variable)

[bryceitoc9]

Documenting a workaround (ensuring the default region is set in config) here: #424

[justinmk3]

Implemented in https://github.com/aws/aws-toolkit-vscode-staging/pull/1690

Released as part of #5902