Show inline completions in menu only for supported providers (#22181)

This changes #22093 by making the change only have an effect for the
supported provider: Zeta.

Made the change because the UX is still experimental and I don't want to
break existing workflows for Copilot/Supermaven users.

Even Zeta users can opt-out of it by setting
`"show_inline_completions_in_menu": false` in their settings, in case
they want the old show-inline-completion-or-show-lsp-completion behavior
back.

Release Notes:

- N/A

crates/inline_completion/src/inline_completion.rs

@@ -20,6 +20,7 @@ pub struct InlineCompletion {
 pub trait InlineCompletionProvider: 'static + Sized {
     fn name() -> &'static str;
     fn display_name() -> &'static str;
+    fn show_completions_in_menu() -> bool;
     fn is_enabled(
         &self,
         buffer: &Model<Buffer>,
@@ -59,6 +60,7 @@ pub trait InlineCompletionProviderHandle {
         cursor_position: language::Anchor,
         cx: &AppContext,
     ) -> bool;
+    fn show_completions_in_menu(&self) -> bool;
     fn refresh(
         &self,
         buffer: Model<Buffer>,
@@ -95,6 +97,10 @@ where
         T::display_name()
     }
 
+    fn show_completions_in_menu(&self) -> bool {
+        T::show_completions_in_menu()
+    }
+
     fn is_enabled(
         &self,
         buffer: &Model<Buffer>,