fix(meos): proto extra_fields + Werror unused-param in aggregations

Two adjacent compile-breakers found while validating the codegen output of
PR MobilityDB#21 against the latest mariana/main:

1. SerializableAggregationFunction proto declares only {type, on_field,
   as_field}. The 5 MEOS aggregations landing in MobilityDB#16/MobilityDB#17 read additional
   fields out of the proto (vidA/vidB/dMeet/...), so they need the extra
   field. Adds:

       repeated SerializableFunction extra_fields = 4;

   Backwards-compatible (tag 4, new repeated). Aggregations whose extra
   fields are absent continue to deserialize unchanged.

2. CrossDistance/PairMeeting/TemporalLength aggregations carry an unused
   PipelineMemoryProvider& parameter on lower(). Werror=-Wunused-parameter
   turns that into a build failure. Annotates the parameter [[maybe_unused]]
   at the call site — no behavior change, intent stays visible to readers
   who later wire memory into the lowering.

Verified locally on the mobilitynebula-v2 dev image (MEOS baked in):

    cmake --build build-w1 --target nes-physical-operators -j 4
    → [110/111] Linking libnes-physical-operators-registry.a
    → [111/111] Linking libnes-physical-operators.a

Stacks on MobilityDB#21 only because that is the active codegen branch where the
breakage surfaced; the diff itself is independent of any codegen output.

Author: estebanzimanyi

grpc/SerializableVariantDescriptor.proto

@@ -66,6 +66,7 @@ message SerializableAggregationFunction {
     string type = 1;
     SerializableFunction on_field = 2;
     SerializableFunction as_field = 3;
+    repeated SerializableFunction extra_fields = 4;
 }
 
 message AggregationFunctionList {

nes-physical-operators/src/Aggregation/Function/Meos/CrossDistanceAggregationPhysicalFunction.cpp

@@ -116,7 +116,7 @@ void CrossDistanceAggregationPhysicalFunction::combine(
 }
 
 Nautilus::Record CrossDistanceAggregationPhysicalFunction::lower(
-    const nautilus::val<AggregationState*> aggregationState, PipelineMemoryProvider& pipelineMemoryProvider)
+    const nautilus::val<AggregationState*> aggregationState, [[maybe_unused]] PipelineMemoryProvider& pipelineMemoryProvider)
 {
     MEOS::Meos::ensureMeosInitialized();
 

nes-physical-operators/src/Aggregation/Function/Meos/PairMeetingAggregationPhysicalFunction.cpp

@@ -115,7 +115,7 @@ void PairMeetingAggregationPhysicalFunction::combine(
 }
 
 Nautilus::Record PairMeetingAggregationPhysicalFunction::lower(
-    const nautilus::val<AggregationState*> aggregationState, PipelineMemoryProvider& pipelineMemoryProvider)
+    const nautilus::val<AggregationState*> aggregationState, [[maybe_unused]] PipelineMemoryProvider& pipelineMemoryProvider)
 {
     MEOS::Meos::ensureMeosInitialized();
 

nes-physical-operators/src/Aggregation/Function/Meos/TemporalLengthAggregationPhysicalFunction.cpp

@@ -109,7 +109,7 @@ void TemporalLengthAggregationPhysicalFunction::combine(
 }
 
 Nautilus::Record TemporalLengthAggregationPhysicalFunction::lower(
-    const nautilus::val<AggregationState*> aggregationState, PipelineMemoryProvider& pipelineMemoryProvider)
+    const nautilus::val<AggregationState*> aggregationState, [[maybe_unused]] PipelineMemoryProvider& pipelineMemoryProvider)
 {
     MEOS::Meos::ensureMeosInitialized();