Add RDJson support. (#11682)

## Summary

Implement support for RDJson output for `ruff check`, as requested in
#8655.

## Test Plan

Tested using a snapshot test. Same approach as for e.g. the JSON output
formatter.

## Additional info

I tried to keep the implementation close to the JSON implementation.

I had to deviate a bit to make the `suggestions` key work: If there are
no suggestions, then setting `suggestions` to `null` is invalid
according to the JSONSchema. Therefore, I opted for a slightly more
complex implementation, that skips the `suggestions` key entirely if
there are no fixes available for the given diagnostic. Maybe it would
have been easier to set `"suggestions": []`, but I ended up doing it
this way.

I didn't consider notebooks, as I _think_ that RDJson doesn't work with
notebooks. This should be confirmed, and if so, there should be some
form of warning or error emitted when trying to output diagnostics for a
notebook.

I also didn't consider `ruff format`, as this comment:
https://github.com/astral-sh/ruff/issues/8655#issuecomment-1811446160
suggests that that wouldn't be compatible.

I'm new to Rust, any feedback is appreciated. 🙂 I
implemented this in order to have a productive rainy saturday afternoon,
I'm not knowledgeable about RDJson beyond the sources linked in the
issue.

crates/ruff_linter/src/settings/types.rs

@@ -515,6 +515,7 @@ pub enum SerializationFormat {
     Github,
     Gitlab,
     Pylint,
+    Rdjson,
     Azure,
     Sarif,
 }
@@ -532,6 +533,7 @@ impl Display for SerializationFormat {
             Self::Github => write!(f, "github"),
             Self::Gitlab => write!(f, "gitlab"),
             Self::Pylint => write!(f, "pylint"),
+            Self::Rdjson => write!(f, "rdjson"),
             Self::Azure => write!(f, "azure"),
             Self::Sarif => write!(f, "sarif"),
         }