Add notion of Observables and Expectation Values #83
Description
Currently, we can write a circuit and return the bit-vector of the qubit measurements. In general, due to the probabilistic nature of quantum computing, one is more interested in the expectation values of a circuit.
On a quantum computer, due to different kinds of (probabilistic) errors this would entail to do a shot-based execution, executing the circuit multiple times to compute the expectation values.
In simulators, this most often is not required due to the ability to access the state vector of the circuit; simulators often do not account for error probabilities of operations, thus a single execution reflects the correct variance.
The QILLR dialect, due to its relation to the QIR backend, already started something via #22. This must be scoped correctly in the quantum dialect.
Furthermore, often pauli strings are used define the kind of measurement which is used to build the expectation values. However, the pauli strings should not be optimized in a way a normal Rx gate is. This maybe should require a different operation (and type?).