Add Infracost Terraform Provider for modelling usage based costs

[aliscott]

This is a proposal for adding an Infracost Terraform Provider for representing usage based costs. This would allow us to reference any existing Terraform resource and augment it with additional information we can use to calculate the usage costs.

Here is an example of how this could be represented for a lambda function:

resource "infracost_usage_aws_lambda_function" "test_lambda" {
  resources = [aws_lambda_function.test_lambda]

  estimated_request_count {
    min = 10000
    max = 15000
  } 
  estimated_request_duration {
    min = 260
    max = 300
  }
}

This would also require a min/max cost range to be represented as first suggested in aliscott#21.

Other options for this could include an additional config file (as suggested in aliscott#34). Other suggestions are welcome.

[aliscott]

Initial work has been done and released in https://github.com/infracost/infracost/releases/tag/v0.5.0. Provider repo is https://github.com/infracost/terraform-provider-infracost.