Merge HTTPManagerRetryBehavior.CustomStrategy into HTTPManagerRetryBehavior.Strategy #39
Description
We should turn HTTPManagerRetryBehavior.Strategy into a struct so we can merge the custom retry behavior into it.
This could alternatively be done with a new .custom case, but by turning it into a struct we allow ourselves to add new built-in strategies in the future in a backwards-compatible fashion.
This is a backwards-incompatible change.