Support general matrix elements in backend

[obackhouse]

Description

Adds a function to CuTensorNetBackend to get general matrix elements. Expectation values are computed as a special case of that function.

I've not added tests as of now, as my understanding as per @PabloAndresCQ is that this is soon to be refactored anyway, so this PR may become defunct.

Related issues

Please mention any github issues addressed by this PR.

Checklist

• [ ] I have run the tests on a device with GPUs.
• [ ] I have performed a self-review of my code.
• [ ] I have commented hard-to-understand parts of my code.
• [ ] I have made corresponding changes to the public API documentation.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have updated the changelog with any user-facing changes.