Add memory estimates for plan nodes and sinks with missing estimates

Many plan nodes and sinks, e.g. KuduScanNode, KuduTableSink, ExchangeNode, etc are missing memory estimates entirely.

We should add a basic estimate for all these cases based on experiments and data from real workloads. In some cases 0 may be the right estimate (e.g. for streaming nodes like SelectNode that just pass through data) but we should remove TODOs and document the reasoning in those cases.