feat(codegen): generate paginators and waiters with default parameters for all optional inputs

Issue \

n/a

feat: We generate service client operation methods with a default parameter whenever the input shape to the operation has all optional members. This PR adds the same default parameter convenience for paginators and waiters.
fix: Fix test utility to actually return null when no index is found (indexOf returns -1 when not found, our test logic was predicating off null and silently passing tests that should fail).
fix: https://github.com/awslabs/smithy-kotlin/pull/951 added API settings for internal vs external visibility but we missed paginators and waiters as part of this, updated codegen to respect the visibility for generated paginators and waiters.

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