* updated billing.tf file to conditionally creates billing sink if the billing account resides out of org
* replace resources with modules
* replace individual iam resources with billing_iam local passed in the module
* update module name and path and move iam from local to module call
* update README.md
* Add log_bucket option to billing_account variable, and update billing account IAM assignments
* update 0-bootstrap README to reflect changes to billing account module
* Update current bootstrap tests to reflect the change to billing_account variable
* Create test for the case when billing account log bucket is created
* running fmt
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Co-authored-by: Ludovico Magnocavallo <ludomagno@google.com>