Add a memory-based trigger for MSE hash aggregation spill

[wirybeaver]

Motivation

MSE hash aggregation spill currently uses mseAggregationSpillMaxGroups as a proxy for retained memory. Memory per group varies greatly across aggregation functions (e.g. SUM versus DISTINCTCOUNT/sketch functions) and the key representation. A single group-count value cannot provide a reliable memory bound or avoid unnecessary I/O across these queries.

Proposed direction

• Introduce an estimated retained-bytes signal for group keys and aggregation intermediate states. Define the estimate contract for AggregationFunction (including variable-size functions and plugin implementations) without breaking existing plugins.
• Spill when the estimated memory budget or mseAggregationSpillMaxGroups is exceeded; name the new query option by its units (for example, mseAggregationSpillMaxBytes).
• Evaluate the trigger during block processing when variable-size states can grow significantly within one input block.
• Preserve numGroupsLimit as a per-hash-table hard ceiling and keep restore-side group limits distinct from the input spill trigger.
• Test memory estimation, mixed aggregate types, skewed keys, and the interaction of both triggers.

This is a follow-up to roadmap #19727: retained-memory-based triggering is outside the current four-PR, group-count-based spill scope and does not block it. Disk-byte quotas are separate from this memory budget.