Add matrix_opt for cuSPARSE backend

[tdehoff]

Summary:
Adds the following for the cuSPARSE backend: matrix_handle for CSR and CSC formats, SpMM, small updates to SpMV, draft of SpGEMM.

Details:

Since I wasn't able to test SpGEMM, I expect it to have multiple issues.
A simple case of SpMM was verified in the example file.

Merge Checklist:

• Passing CI
• Update documentation or README.md
• Additional Test/example added (if applicable) and passing
• At least one reviewer approval
• (optional) Clang sanitizer scan run and triaged
• Clang formatter applied (verified as part of passing CI)

[yhmtsai]

Hi @tdehoff
Excellent work! I have some nit which needs to address.
When it is possible, we would like to have a small pr rather than large pr.
Thus, I will suggest to remove the spgemm part out of this pr because it is not finished and can be in another pr like #54

[tdehoff]

Hi @yhmtsai!
Thank you for the feedback. I renamed all instances of matrix_handle to matrix_descriptor, removed the unused b_descr and c_descr, and removed spgemm from the PR.
Let me know if there's anything else I missed!