Support for `Sum` observable type in `Expectation`, `Variance`, `AdjointGradient` result types for `shots=0`

**Describe the feature you'd like**
Currently, Amazon Braket's on-demand state vector simulator only supports the `Sum` observable type for `AdjointGradient` results. However, this is a natural construct for `Expectation` and `Variance` when `shots=0` (for exact, non-sampled workloads).

**How would this feature be used? Please describe.**
Users would be able to construct `Sum` observables and measure them in `Expectation` and `Variance` when `shots=0`.

**Describe alternatives you've considered**
Right now, users have to explicitly create `Expectation`/`Variance`s for each term in the sum and combine them by hand.

**Additional context**
This is another opportunity to carefully use dynamic threading for smaller circuits for a performance improvement.