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/collab/Cargo.toml

@@ -31,10 +31,12 @@ collections.workspace = true
 dashmap = "5.4"
 envy = "0.4.2"
 futures.workspace = true
+google_ai.workspace = true
 hex.workspace = true
 live_kit_server.workspace = true
 log.workspace = true
 nanoid = "0.4"
+open_ai.workspace = true
 parking_lot.workspace = true
 prometheus = "0.13"
 prost.workspace = true
@@ -80,7 +82,6 @@ git = { workspace = true, features = ["test-support"] }
 gpui = { workspace = true, features = ["test-support"] }
 indoc.workspace = true
 language = { workspace = true, features = ["test-support"] }
-lazy_static.workspace = true
 live_kit_client = { workspace = true, features = ["test-support"] }
 lsp = { workspace = true, features = ["test-support"] }
 menu.workspace = true