Support Terraform to fetch the EC2 metadata and authorize with the identity

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

This is to support Terraform Extension for Azure DevOps to create AWS resources. We are using self-hosted agents running as EC2 in the account we want Terraform to act in. Is there a way to get Terraform to use the EC2 Instance Metadata of the host EC2 instead of a service connection? -- This is related to an open issue on [azure-pipelines-extension](https://github.com/microsoft/azure-pipelines-extensions/issues/839)

**Describe the solution you'd like**

Pulling in the comment @hunterwerlla made so we can track it here:

"For the AWS Toolkit for Azure DevOps we support this use case by making the authentication optional. It would make sense to make provider optional/add a noop "Environment" option, so if the machine already has credentials, they can be used."

**Describe alternatives you've considered**

**Additional context**

Start by locating the Terraform extension's provider and service-connection authentication entry points, then review the linked azure-pipelines-extensions issue for related context. Determine how an optional provider or Environment option could use EC2 instance metadata, and define completion as Terraform authenticating on self-hosted EC2 agents without a service connection.

Written by the indexing model from the issue text.