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>

docs/src/configuring_zed.md

@@ -606,28 +606,6 @@ These values take in the same options as the root-level settings with the same n
 
 `boolean` values
 
-## Semantic Index
-
-- Description: Settings related to semantic index.
-- Setting: `semantic_index`
-- Default:
-
-```json
-"semantic_index": {
-  "enabled": false
-},
-```
-
-### Enabled
-
-- Description: Whether or not to display the `Semantic` mode in project search.
-- Setting: `enabled`
-- Default: `true`
-
-**Options**
-
-`boolean` values
-
 ## Show Call Status Icon
 
 - Description: Whether or not to show the call status icon in the status bar.