Add job for integration tests

Add `build` stages

- `cargo_ping`: Send a ping to a Cargo server
- `cargo_shutdown`: Send a shutdwon request to a Cargo server

- Add `transfer_state` enum with appropriate values for a transfer
  current status.
- Add `transfer_status` class with functions `state()`, `done()`,
  `failed()` and `error()` to retrieve a transfer's current status and
  its associated error condition in case of failure.
- Extend the `transfer` class with functions `wait()`, and `wait_for()`,
  to wait for the completion of a transfer request.
- Add new error codes: `no_such_transfer` and `transfer_in_progress`.
- Add new formatters: `transfer_state` and `std::optional<T>`.
- Add new API function: `transfer_dataset` as a convenience function to
  request the transfer of a single dataset.

Also, enclose classes in `cargo` namespace

RPC handlers are now member functions of `master_server`

Merge branch 'amiranda/11-use-thallium-providers-for-rpc-handlers-instead-of-free-functions' into 'main'

Resolve "Use thallium providers for RPC handlers instead of free functions."

Closes #11

See merge request !7

RPC handlers are now member functions of `master_server`

Remove boost::program_options

Resolve "Cargo +Tests Build failure with Spack"

Closes #9

See merge request !5