Rollup merge of #138934 - onur-ozkan:extended-config-profiles, r=Kobzol

support config extensions

_Copied from the `rustc-dev-guide` addition:_

>When working on different tasks, you might need to switch between different bootstrap >configurations.
>Sometimes you may want to keep an old configuration for future use. But saving raw config >values in
>random files and manually copying and pasting them can quickly become messy, especially if >you have a
>long history of different configurations.
>
>To simplify managing multiple configurations, you can create config extensions.
>
>For example, you can create a simple config file named `cross.toml`:
>
>```toml
>[build]
>build = "x86_64-unknown-linux-gnu"
>host = ["i686-unknown-linux-gnu"]
>target = ["i686-unknown-linux-gnu"]
>
>
>[llvm]
>download-ci-llvm = false
>
>[target.x86_64-unknown-linux-gnu]
>llvm-config = "/path/to/llvm-19/bin/llvm-config"
>```
>
>Then, include this in your `bootstrap.toml`:
>
>```toml
>include = ["cross.toml"]
>```
>
>You can also include extensions within extensions recursively.
>
>**Note:** In the `include` field, the overriding logic follows a right-to-left order. For example,
in `include = ["a.toml", "b.toml"]`, extension `b.toml` overrides `a.toml`. Also, parent extensions
always overrides the inner ones.

try-job: x86_64-mingw-2

Author: ChrisDenton