in src/main/java/software/amazon/msk/auth/iam/internals/MSKCredentialProvider.java [193:201]
STSAssumeRoleSessionCredentialsProvider createSTSRoleCredentialProvider(String roleArn,
String sessionName, String stsRegion) {
AWSSecurityTokenService stsClient = AWSSecurityTokenServiceClientBuilder.standard()
.withRegion(stsRegion)
.build();
return new STSAssumeRoleSessionCredentialsProvider.Builder(roleArn, sessionName)
.withStsClient(stsClient)
.build();
}