Add base representation of SparseObservable

This adds the base representation of `SparseObservable`, including the
simple constructors from Python space and the ability to view the data
buffers.

This commit does not include the mathematical manipulations of the
operators, nor some of the helper methods that will be used to
manipulate the operators in the context of primitives execution. These
will follow in subsequent patches.

The design and implementation notes of `SparseObservable` are described
in a Qiskit RFC that preceeded this patch series[^1], and it's best to
consult that document for full details on the operator considerations.

[^1]: https://github.com/Qiskit/RFCs/blob/7a74b08793475b7b0142d3a3f7142cabcfd33ab8/0021-sparse-observable.md

crates/accelerate/Cargo.toml

@@ -26,6 +26,7 @@ qiskit-circuit.workspace = true
 thiserror.workspace = true
 ndarray_einsum_beta = "0.7"
 once_cell = "1.20.1"
+bytemuck.workspace = true
 
 [dependencies.smallvec]
 workspace = true
@@ -60,4 +61,4 @@ version = "0.18.22"
 features = ["macro"]
 
 [features]
-cache_pygates = ["qiskit-circuit/cache_pygates"]
+cache_pygates = ["qiskit-circuit/cache_pygates"]

crates/accelerate/src/lib.rs

@@ -39,6 +39,7 @@ pub mod remove_diagonal_gates_before_measure;
 pub mod results;
 pub mod sabre;
 pub mod sampled_exp_val;
+pub mod sparse_observable;
 pub mod sparse_pauli_op;
 pub mod split_2q_unitaries;
 pub mod star_prerouting;