Add an option for custom toolchain and interactive toolchain selection

[playfulFence]

closes #250

I've created a new category for the tui template with a placeholder option, which then will be replaced by installed toolchains.

Toolchain list in the TUI is auto-populated from installed rustup toolchains that actually support the selected chip target and current MSRV of a template project

The Toolchain: label in the TUI header shows either the user’s chosen toolchain or that template defaults will be used, this is how workflow works too; if user selected different toolchain in the TUI or via --toolchain parameter - selected one will be used, otherwise template defaults from rust-toolchain.yaml

compress-demo.mp4

[SergioGasquez]

We should filter out the channels that do not meet the msrv

[playfulFence]

Sergio just confirmed: it works now

[SergioGasquez]

Thanks for working on this! Just left some comments, maybe we should have a toolchain module as we have quite a lot of logic for it.

[playfulFence]

@SergioGasquez, I agree with a separate module part, done

[MabezDev]

Thanks!

[bjoernQ]

LGTM