Introduce staff-only inline completion provider (#21739)

Release Notes:

- N/A

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>
Co-authored-by: Bennet <bennet@zed.dev>
Co-authored-by: Thorsten <thorsten@zed.dev>

crates/rpc/src/llm.rs

@@ -33,3 +33,14 @@ pub struct PerformCompletionParams {
     pub model: String,
     pub provider_request: Box<serde_json::value::RawValue>,
 }
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct PredictEditsParams {
+    pub input_events: String,
+    pub input_excerpt: String,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct PredictEditsResponse {
+    pub output_excerpt: String,
+}