- 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

Resolve "Failed to checkout tag for `genopts` and for `expected`"

Closes #7

See merge request !4

Resolve "Don't rely on backend prefix to determine parallel transfers"

Closes #5

See merge request !3

Resolve "CMake's `list(PREPEND ...)` command requires CMake >= 3.15"

Closes #2 and #3

See merge request !2

Resolve "Add support for Spack installation"

Closes #1

See merge request !1