feature - implement RFC 023 approximate count distinct (#40)

[dannymeijer]

Summary

• adds RFC 023's first approximate/sketch slice with approx_count_distinct(...)
• adds registry approximation metadata, aggregate modifier policy, and Substrait extension mapping for the helper
• keeps DataFusion-specific naming inside the DataFusion adapter boundary while preserving the InQL/Substrait function name
• documents the approximate function family and marks RFC 023 as in progress

Verification

• make fmt
• incan test tests/test_approximate_functions.incn
• incan test tests/test_function_registry.incn
• incan test tests/test_substrait_plan.incn
• incan test tests/test_prism.incn
• incan test tests/test_session_aggregates.incn
• make fmt-check
• make test-style
• make registry-metadata
• make build
• make test
• make smoke-consumer
• make pre-commit

Stack / Merge Order

This PR is stacked after #49 and should merge only after:

#44 -> #45 -> #46 -> #47 -> #48 -> #49 -> this PR

Refs #40