add: RFC for user-configurable retry behavior (#732)

* add: incomplete draft rfc for retry configuraion

* update: rfc based on suggestions
add: reference to SDK reference page on max_attempts
fix: heading levels
update: simplify shared config example
fix: copypaste error in EnvironmentVariableRegionProvider

* update: to use builder method called max_attempts

* fix: leftover language

* update: go back to RetryConfig
add: more definitions
update: examples
add: explainer for process of sending a request
add: note about breaking change to
update: changes checklist
add: note about breaking changes

* fix: use correct module for shard config
update: emit warn log for invalid AWS_MAX_ATTEMPTS

* add: note about where RetryConfigDecorator will be defined
add: note about panics when setting invalid max_attempts