Support shots vectors in `execute` and `batch_execute`.

**Describe the feature you'd like**
Support for running `QuantumTape`s with different shot amounts in one fell swoop.

**How would this feature be used? Please describe.**
Would allow Braket to run tapes with multiple shot counts specified, which currently isn't supported.

**Describe alternatives you've considered**
We could continue to not support this.

**Additional context**
Shot vectors are already supported by [PL's own devices](https://docs.pennylane.ai/en/stable/code/api/pennylane.devices.default_qubit.DefaultQubit.html?highlight=shot_vector#pennylane.devices.default_qubit.DefaultQubit.shot_vector) and their interface with Qiskit.