feat: enable opt-in legacy authentication flow

[jar-b]

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Adds an opt-in "legacy" workflow to support the Terraform core S3 backend upgrade which will span multiple minor versions.

$ go test -count=1 ./...
?       github.com/hashicorp/aws-sdk-go-base/v2/internal/awsconfig      [no test files]
?       github.com/hashicorp/aws-sdk-go-base/v2/internal/config [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 5.441s
ok      github.com/hashicorp/aws-sdk-go-base/v2/diag    0.296s
ok      github.com/hashicorp/aws-sdk-go-base/v2/internal/endpoints      0.188s
ok      github.com/hashicorp/aws-sdk-go-base/v2/internal/expand 0.237s
ok      github.com/hashicorp/aws-sdk-go-base/v2/logging 0.225s
ok      github.com/hashicorp/aws-sdk-go-base/v2/tfawserr        0.289s
ok      github.com/hashicorp/aws-sdk-go-base/v2/useragent       0.336s

[gdavison]

Looks good! 🚀