A part of #44690.

Some tools (rustfmt and clippy) used in tool attributes are hardcoded in the compiler.
We need some way to introduce them without hardcoding as well.

#66070 introduced a way to do it with a crate level attribute:

#![register_tool(my_tool)]

#[my_tool::anything] // OK
fn main() {}

The previous attempt to introduce them through command line (#57921) met some resistance.

This probably needs to go through an RFC before stabilization.