Allow customizing waiter retry strategy

[ianbotsf]

Currently, waiters are codegenned with a static retry strategy using either hardcoded defaults (scale factor, jitter, max attempts) or model-derived defaults (initial delay, max backoff). This is inflexible and does not allow users to customize the waiter parameters in situations where they may desire to do so.

Define a mechanism that allows callers to specify a non-default retry strategy while still defaulting to the currently defined strategies.

[ianbotsf]

Resolving in favor of #839.