sso: GetRoleCredentialsResponse_roleCredentials is required

[endgame]

It's the only field in the response, and @pbrisbin 's work makes me think this is always present in a successful response.

See also: botocore:

https://github.com/boto/botocore/blob/cecca8c0e916297baeb06d236b2b23060170d8bb/botocore/data/sso/2019-06-10/service-2.json#L151-L154

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