Support accumulator initializer expression in HashAgg

Each accumulator has an initializer expression that can be used to initialize an accumulator in certain scenarios. One example of this is when DSGroup creates a fresh group, and another is when DSGroup reads spilled accumulator states from disk and re-initializes groups.

We need a mechanism to initialize agg expressions in sbe. This could be done by augmenting HashAgg to evaluate an initializer expression.

We can think of other ways as well.