Add `optional` key to components.toml

With some plugins now separate from the main Sway repo we should think about which binaries should belong to the default toolchain installation.

We should introduce a new `optional` key that is a boolean:

`false` = plugin is distributed within an official toolchain by default. A plugin with `optional = false` is installed in `fuelup toolchain install latest`.
`true` = plugin is not distributed within an official toolchain. For example, if `forc-explorer` is an optional plugin, users will have to specifically do `fuelup component add forc-explorer` to add it to a toolchain.

To accomplish this, there are some logic updates to adding/removing components needed to make this work, since official toolchains do not currently support that action. `components.toml` and `components.rs` should also be updated to handle the new `optional` key.

To start off, `forc` and `fuel-core` should definitely not be optional components. `forc-wallet` is contentious, but I think `forc-explorer` can be an optional component.