Allow AI interactions to be proxied through Zed's server so you don't need an API key (#7367)

Co-authored-by: Antonio <antonio@zed.dev>

Resurrected this from some assistant work I did in Spring of 2023.
- [x] Resurrect streaming responses
- [x] Use streaming responses to enable AI via Zed's servers by default
(but preserve API key option for now)
- [x] Simplify protobuf
- [x] Proxy to OpenAI on zed.dev
- [x] Proxy to Gemini on zed.dev
- [x] Improve UX for switching between openAI and google models
- We current disallow cycling when setting a custom model, but we need a
better solution to keep OpenAI models available while testing the google
ones
- [x] Show remaining tokens correctly for Google models
- [x] Remove semantic index
- [x] Delete `ai` crate
- [x] Cloud front so we can ban abuse
- [x] Rate-limiting
- [x] Fix panic when using inline assistant
- [x] Double check the upgraded `AssistantSettings` are
backwards-compatible
- [x] Add hosted LLM interaction behind a `language-models` feature
flag.

Release Notes:

- We are temporarily removing the semantic index in order to redesign it
from scratch.

---------

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/rpc/src/proto.rs

@@ -149,7 +149,10 @@ messages!(
     (CallCanceled, Foreground),
     (CancelCall, Foreground),
     (ChannelMessageSent, Foreground),
+    (CompleteWithLanguageModel, Background),
     (CopyProjectEntry, Foreground),
+    (CountTokensWithLanguageModel, Background),
+    (CountTokensResponse, Background),
     (CreateBufferForPeer, Foreground),
     (CreateChannel, Foreground),
     (CreateChannelResponse, Foreground),
@@ -160,6 +163,7 @@ messages!(
     (DeleteChannel, Foreground),
     (DeleteNotification, Foreground),
     (DeleteProjectEntry, Foreground),
+    (EndStream, Foreground),
     (Error, Foreground),
     (ExpandProjectEntry, Foreground),
     (ExpandProjectEntryResponse, Foreground),
@@ -211,6 +215,7 @@ messages!(
     (JoinProjectResponse, Foreground),
     (JoinRoom, Foreground),
     (JoinRoomResponse, Foreground),
+    (LanguageModelResponse, Background),
     (LeaveChannelBuffer, Background),
     (LeaveChannelChat, Foreground),
     (LeaveProject, Foreground),
@@ -300,6 +305,8 @@ request_messages!(
     (Call, Ack),
     (CancelCall, Ack),
     (CopyProjectEntry, ProjectEntryResponse),
+    (CompleteWithLanguageModel, LanguageModelResponse),
+    (CountTokensWithLanguageModel, CountTokensResponse),
     (CreateChannel, CreateChannelResponse),
     (CreateProjectEntry, ProjectEntryResponse),
     (CreateRoom, CreateRoomResponse),