language_model: Denote the availability of language models (#15660)

This PR updates the `LanguageModel` trait with a new method for denoting
the availability of a model.

Right now we have two variants:

- `Public` for models that have no additional restrictions (other than
their respective setup/authentication requirements)
- `RequiresPlan` for models that require a specific Zed plan

Release Notes:

- N/A

crates/language_model/src/provider/cloud.rs

@@ -18,7 +18,7 @@ use std::{future, sync::Arc};
 use strum::IntoEnumIterator;
 use ui::prelude::*;
 
-use crate::LanguageModelProvider;
+use crate::{LanguageModelAvailability, LanguageModelProvider};
 
 use super::anthropic::count_anthropic_tokens;
 
@@ -236,6 +236,10 @@ impl LanguageModel for CloudLanguageModel {
         format!("zed.dev/{}", self.model.id())
     }
 
+    fn availability(&self) -> LanguageModelAvailability {
+        self.model.availability()
+    }
+
     fn max_token_count(&self) -> usize {
         self.model.max_token_count()
     }