custom toolchain: add `nightly` component

It might be useful to support adding the current nightly of a component to a custom toolchain, eg. `fuelup component add forc@nightly`. I can foresee this use case if for example someone wants to test the `nightly` version of a component against a `latest` version.

In future we can also support adding a nightly at a specific date but that should come later.

Start at the CLI entry point for `fuelup component add`, focusing on custom toolchain component version handling. Verify the requested `forc@nightly` form and compare it with the existing `latest` behavior; done means a custom toolchain can contain the current nightly component alongside a latest component.

Written by the indexing model from the issue text.