`rustup component add` should install multiple components at once

[rami3l]

We want rustup component add to behave more like #3018.

So far the process is still explicitly sequential:

rustup/src/toolchain/distributable.rs

Lines 63 to 64 in 6163a00

	pub(crate) async fn add_component(&self, mut component: Component) -> anyhow::Result<()> {
	// TODO: take multiple components?