Make retry policy pluggable/extensible - similar to Java SDK

The Java 2 SDK allows the retry policy to be pluggable. This is important because it puts the policy in the hands of the user and since each policy type (fail fast, retry until timeout, etc) has it's tradeoffs, the user can chose the policy which best fits their use-case(s).

Additionally, by making it pluggable/extensible it's possible for a user to provide their own policy which fits their own use-case(s), if an existing one doesn't meet their requirements.