collab: Add support for more providers to the LLM service (#15832)

This PR adds support for additional providers to the LLM service:

- OpenAI
- Google
- Custom Zed models (through Hugging Face)

Release Notes:

- N/A

crates/rpc/src/llm.rs

@@ -2,7 +2,18 @@ use serde::{Deserialize, Serialize};
 
 pub const EXPIRED_LLM_TOKEN_HEADER_NAME: &str = "x-zed-expired-token";
 
+#[derive(Debug, PartialEq, Eq, Clone, Copy, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum LanguageModelProvider {
+    Anthropic,
+    OpenAi,
+    Google,
+    Zed,
+}
+
 #[derive(Serialize, Deserialize)]
 pub struct PerformCompletionParams {
+    pub provider: LanguageModelProvider,
+    pub model: String,
     pub provider_request: Box<serde_json::value::RawValue>,
 }