Support the toolchain pattern, starting with latest toolchain

[eightfilms]

fuelup is largely inspired by rustup in terms of how it should distribute packages in the Fuel toolchain in future, and this inspiration extends to the toolchain concept.

Following a discussion here, a good first step towards building the foundation of toolchain management would be to support the latest toolchain (which is basically what we have now!)

This would mean:

• We have some kind of config file within ~/.fuelup/bin, this could be in the form of a settings.toml similar to rustup, where configs (eg. the default toolchain) are set,
• We download bins to /toolchains/latest/bin instead, so that next time we can download other versions to, for example, /toolchains/stable/bin, or we can even support custom toolchains i.e. /toolchains/my-toolchain/bin

[eightfilms]

In making this work we need to support the proxy pattern - it means we will run forc and fuel-core through fuelup, pass necessary args through fuelup to the respectively tools. This is how rustup is aware of which toolchain to use for which tool.