[red-knot] User-level configuration (#16021)

## Summary

This PR adds support for user-level configurations
(`~/.config/knot/knot.toml`) to Red Knot.

Red Knot will watch the user-level configuration file for changes but
only if it exists
when the process start. It doesn't watch for new configurations, 
mainly to simplify things for now (it would require watching the entire
`.config` directory because the `knot` subfolder might not exist
either).

The new `ConfigurationFile` struct seems a bit overkill for now but I
plan to use it for
hierarchical configurations as well. 


Red Knot uses the same strategy as uv and Ruff by using the etcetera
crate.

## Test Plan

Added CLI and file watching test

crates/ruff_db/src/system.rs

@@ -1,7 +1,12 @@
 pub use glob::PatternError;
 pub use memory_fs::MemoryFileSystem;
+
+#[cfg(all(feature = "testing", feature = "os"))]
+pub use os::testing::UserConfigDirectoryOverrideGuard;
+
 #[cfg(feature = "os")]
 pub use os::OsSystem;
+
 use ruff_notebook::{Notebook, NotebookError};
 use std::error::Error;
 use std::fmt::Debug;