Physical-plan serialization drops ProjectionExec metadata

[gene-bordegaray]

Physical-plan serialization

Before ProjectionExecNode serialized:

PhysicalPlanNode input = 1;
repeated PhysicalExprNode expr = 2;
repeated string expr_name = 3;

On decode, DataFusion reconstructed the projection which is fine when the complete output schema can be derived from the expression and input but metadata cannot be.

Given:

Before serialization:

Input field:
  value: Int32
  metadata: {}

Projection expression:
  value@0 AS value

ProjectionExec output schema:
  value: Int32
  field metadata: {"field-key": "field-value"}
  schema metadata: {"schema-key": "schema-value"}

---

After serialization and decoding:

ProjectionExec output schema:
  value: Int32
  field metadata: {}
  schema metadata: {}

---

this was roginally found in #24670 but not fixed there since it was out of optimizer scope / unrelated bug