Update OSCAL Component Definition for Istio to include 800-53 controls and explanations

This PR update BB's OSCAL definition to include additional controls as well as better explain the existing controls that Istio implements.

The content in the YAML is a copy of a document that will be published in the next month or two pending review. The goal is to provide an in-depth explanation of the controls for teams running BB, and to clearly delineate which controls Istio implements out of the box or with global configuration vs which require configuration from mission teams to implement.