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/language_model.rs

@@ -14,6 +14,7 @@ use gpui::{
 };
 pub use model::*;
 use project::Fs;
+use proto::Plan;
 pub(crate) use rate_limiter::*;
 pub use registry::*;
 pub use request::*;
@@ -32,6 +33,15 @@ pub fn init(
     registry::init(user_store, client, cx);
 }
 
+/// The availability of a [`LanguageModel`].
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum LanguageModelAvailability {
+    /// The language model is available to the general public.
+    Public,
+    /// The language model is available to users on the indicated plan.
+    RequiresPlan(Plan),
+}
+
 pub trait LanguageModel: Send + Sync {
     fn id(&self) -> LanguageModelId;
     fn name(&self) -> LanguageModelName;
@@ -39,6 +49,11 @@ pub trait LanguageModel: Send + Sync {
     fn provider_name(&self) -> LanguageModelProviderName;
     fn telemetry_id(&self) -> String;
 
+    /// Returns the availability of this language model.
+    fn availability(&self) -> LanguageModelAvailability {
+        LanguageModelAvailability::Public
+    }
+
     fn max_token_count(&self) -> usize;
 
     fn count_tokens(