[Flow Control] Align Flow Control configuration and extensibility story with other pluggable layers. #1715
Description
What would you like to be added: This is a two-fold change. (1) All Flow Control extension points should comply with our plugins API. (2) The Flow Control configuration story should align with our existing methods for other layers (text-based config and env var coverage for every flow control configuration value).
Why is this needed: This is a significant usability improvement and makes our extensibility story more consistent across all layers.