Add support for configuring knot in pyproject.toml files (#15493)

## Summary

This PR adds support for configuring Red Knot in the `tool.knot` section
of the project's
`pyproject.toml` section. Options specified on the CLI precede the
options in the configuration file.

This PR only supports the `environment` and the `src.root` options for
now.
Other options will be added as separate PRs.

There are also a few concerns that I intentionally ignored as part of
this PR:

* Handling of relative paths: We need to anchor paths relative to the
current working directory (CLI), or the project (`pyproject.toml` or
`knot.toml`)
* Tracking the source of a value. Diagnostics would benefit from knowing
from which configuration a value comes so that we can point the user to
the right configuration file (or CLI) if the configuration is invalid.
* Schema generation and there's a lot more; see
https://github.com/astral-sh/ruff/issues/15491

This PR changes the default for first party codes: Our existing default
was to only add the project root. Now, Red Knot adds the project root
and `src` (if such a directory exists).

Theoretically, we'd have to add a file watcher event that changes the
first-party search paths if a user later creates a `src` directory. I
think this is pretty uncommon, which is why I ignored the complexity for
now but I can be persuaded to handle it if it's considered important.

Part of https://github.com/astral-sh/ruff/issues/15491

## Test Plan

Existing tests, new file watching test demonstrating that changing the
python version and platform is correctly reflected.

fuzz/fuzz_targets/red_knot_check_invalid_syntax.rs

@@ -118,7 +118,7 @@ fn setup_db() -> TestDb {
         ProgramSettings {
             python_version: PythonVersion::default(),
             python_platform: PythonPlatform::default(),
-            search_paths: SearchPathSettings::new(src_root),
+            search_paths: SearchPathSettings::new(vec![src_root]),
         },
     )
     .expect("Valid search path settings");