Apply rate limits in LLM service (#15997)

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/collab/migrations_llm/20240806182921_create_providers_and_models.sql

@@ -8,7 +8,10 @@ create unique index uix_providers_on_name on providers (name);
 create table if not exists models (
     id serial primary key,
     provider_id integer not null references providers (id) on delete cascade,
-    name text not null
+    name text not null,
+    max_requests_per_minute integer not null,
+    max_tokens_per_minute integer not null,
+    max_tokens_per_day integer not null
 );
 
 create unique index uix_models_on_provider_id_name on models (provider_id, name);