Integrate `sparseir-rust` into `libsparseir` repository

Restructured as follows:

- cxx
  - src
  - include
  - tests (without C-API integration tests)
- rust
  - sparseir-rust
  - sparseir-capi
- python
- julia
- fortran
- capi-integration-tests
  - sample.code...
  - cintegration tests...
- capi-benchmark-tests

In CI, we first run all unit tests for the C++ and Rust backends, then run the C-API integration tests for each backend separately.