catalog: builtin metric sinks types, catalog item, and lowering (SQL-554)

[mtabebe]

Problem:

The Prometheus cluster metrics feature needs the authoring type, catalog representation, and lowering for builtin metric sinks.

Nothing gets installed and there's no runtime behavior change yet.

The abstraction is the generic, backend-agnostic MetricSink, not a Prometheus-specific one.

Solution:

• Add the BuiltinMetricSink authoring type, along with BuiltinMetricSinkValue, MetricSinkKind, a Builtin::MetricSink variant.
• builtin_metric_sink_view_sql projects a sink's query into the canonical MetricSink row shape (metric_name, metric_type, labels, value, help), UNION ALL-ed across the value columns.
• Each sink materializes a companion BuiltinView (the canonical shape, built at enumeration time) plus a thin CatalogItem::MetricSink that points at it.
• Add CatalogItemType::MetricSink (rendered metric-sink) and the CatalogItem::MetricSink, MetricSink, and MetricSinkValue structs, then wire them through every exhaustive match
• Boot construction builds the item directly (create_sql: None)
• For discoverability, the mz_metric_sinks_raw backing table (populated in builtin_table_updates) feeds the mz_internal.mz_metric_sinks view and mz_catalog.mz_objects (as type = 'metric-sink'). Note: It's registered before mz_objects in the builtin list, since a builtin view can only depend on lower-id builtins.
• Register the first concrete sink, mz_metric_dataflow_errors, with companion view mz_metric_dataflow_errors_shaped. No dataflow is installed here.

Testing:

• Unit tests for the lowering
• catalog-protos snapshot consistency tests

Note to reviewers:

The most important things to review are:

• src/catalog/src/builtin.rs — the BuiltinMetricSink types + the concrete mz_metric_dataflow_errors sink
• src/catalog/src/builtin/builtin.rs — the lowering + make_mz_metric_sinks
• src/catalog/src/memory/objects.rs — the CatalogItem::MetricSink modeling
• src/adapter/src/catalog/apply.rs — boot construction
• src/catalog/src/builtin/mz_internal.rs — the mz_metric_sinks_raw table
• src/adapter/src/catalog/builtin_table_updates.rs — pack_metric_sink_update
• src/adapter/src/optimize/metric_sink.rs — lowering tests

There are some mechanical changes (exhaustive-match arms for the new CatalogItem::MetricSink variant across sql/ and adapter/coord/, plus new OIDs) or generated changes (objects_v91.rs, the encoding snapshot, and the regenerated SLT/testdrive golden files)