Add configurable retry.BackoffDelayer

[ewbankkit]

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Closes #1044.

Adds the Backoff field to the Config structure to allow a configurable backoff strategy.

% go test ./...
?   	github.com/hashicorp/aws-sdk-go-base/v2/configtesting	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/awsconfig	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/constants	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/errs	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/slices	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/test	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/mockdata	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/servicemocks	[no test files]
ok  	github.com/hashicorp/aws-sdk-go-base/v2	6.593s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/diag	1.001s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/internal/config	0.815s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/internal/endpoints	1.127s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/internal/expand	1.346s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/logging	1.443s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/tfawserr	1.197s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/useragent	1.233s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/validation	1.291s

[gdavison]

Looks good! 🚀