Support custom profiles to always install certain components

Problem you are trying to solve

I always want the rust-analyzer component installed when working with Rust. I work across a number of rust projects that periodically update their toolchains. Each time this happens, I have to manually run rustup component add rust-analyzer for that new toolchain. I often forget, and am briefly confused when I open an editor and don't get any LSP features.

It seems that right now my only option is to set my default profile to complete, which has many components I don't need, and the docs reccommend against it.

Solution you'd like

I'd like the ability to define a custom profile. It should include a "base" profile (e.g. default) and a list of additional components (e.g. rust-analyzer) to include in that profile.

Ideally this would go in a config file, but based on the docs there is no public config file, so I guess a CLI command like rustup profile [add|remove] would work?

Notes

No response

Start with the rustup configuration documentation linked in the issue and investigate how the existing default and complete profiles are represented and selected. Define how a custom profile would persist its base profile and additional components, then verify that a component such as rust-analyzer is installed when toolchains update.