[services][terraform] Add default ManagedBy tag to all resources

Summary:
Set up a default tag ManagedBy = Terraform to all resources created by Terraform. This will allow us to identify resources created by Terraform and distinguish them from resources created manually.

Depends on D8570

Test Plan: terraform plan shows that the tag will be added to dynamodb tables and s3 buckets.

Reviewers: jon, varun, michal

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8571