[WIP] feat: Add AggregateField and Sum with Query

[KKimj]

#1703

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Type of Change

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• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
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Notes

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[jwinarske]

One thing to fix before this goes further — QueryInternal::Aggregate in
firestore/src/main/query_main.cc:

AggregateQuery QueryInternal::Aggregate(const AggregateField& aggregate_field) {
  // TODO: Implement Aggregate
  // return MakePublic(aggregate_field);
}

That falls off the end of a non-void function, which is UB — with optimization
on you get whatever happens to be in the return slot, and AggregateQuery's
destructor then runs on it. It's also the desktop path, so as it stands this PR
adds SUM for Android and leaves desktop worse than untouched.

The good news is that the desktop implementation is small. The vendored core
already has everything:

api::Query::Aggregate(std::vector<model::AggregateField>&&) const
api::AggregateQuery::GetAggregate(callback<StatusOr<model::ObjectValue>>)
model::AggregateField::OpKind { Sum, Avg, Count }

So Aggregate is a matter of building the AggregateField vector with an
alias, calling query_.Aggregate(...), and reading the result back out of the
ObjectValue by that alias in AggregateQueryInternal::Get — the existing
Get uses the count-specific Get(StatusOr<int64_t>) callback, which won't
serve a sum.

Two details that cost me time: a sum of integers comes back as
integer_value and a sum of doubles as double_value, so you need both; and
a missing alias means the aggregation matched nothing rather than summed to
zero.

I have this working on desktop against the Firestore emulator as a downstream
patch. It's desktop-only and doesn't use the AggregateField public shape this
PR introduces, so it isn't a drop-in, but happy to share it if it's useful.