Adds IAM role chaining

[gdavison]

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Reference: hashicorp/terraform-provider-aws#22728

Adds support for IAM role chaining when assuming roles.

[ewbankkit]

LGTM 🚀.

% 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	13.959s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/diag	0.417s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/internal/config	0.738s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/internal/endpoints	0.993s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/internal/expand	0.560s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/logging	1.118s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/tfawserr	1.611s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/useragent	1.498s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/validation	0.806s

Needs a CHANGELOG entry because of the breaking change to Config.AssumRole.