Rollup merge of #142379 - Stypox:bootstrap-tool-config, r=Kobzol

Add bootstrap option to compile a tool with features

Add an option to specify which features to build a tool with, e.g. it will be useful to build Miri with tracing enabled:
```toml
tool-config.miri.features = ["tracing"]
```

See [this Zulip thread](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Passing.20--features.20to.20Miri.20build.20using.20.2E.2Fx.2Epy/with/523564773) for the options considered. If the final decision will be different than what I wrote now, I will update the code as needed. The reason why the option is `tool-config.miri.features` instead of something like `tool-features.miri` is to possibly allow adding more tool-specific configurations in the future.

I didn't do any validation of the keys of the `tool-config` hashmap, since I saw that no validation is done on the `tools` hashset either.

I don't like much the fact that features can be chosen by various places of the codebase: `Step`s can have some fixed `extra_features`, `prepare_tool_cargo` will add features depending on some bootstrapping options, and the newly added option can also contribute features to tools. However I think it is out of scope of this PR to try to refactor all of that (if it even is refactorable), so I left a comment in the codebase explaining all of the sources of features I could find.

Author: matthiaskrgr