Feature: make retry strategy exponential backoff multiplier configurable (#1548)

* feature: make retry strategy backoff configurable
update: smithy client to respect configurable backoff
update: CHANGELOG.next.toml

* fix: calculate_exponential_backoff

* update: make the backoff calculation simpler
update: default backoff multiplier to be 1 second to better match previous retry behavior

* update: retry builder backoff multiplier default

* rename: backoff_multiplier to initial_backoff
update: calculate_exponential_backoff tests for readability

* remove: copypasted code

* update: CHANGELOG.next.toml