collab: Setup database for LLM service (#15882)

This PR puts the initial infrastructure for the LLM service's database
in place.

The LLM service will be using a separate Postgres database, with its own
set of migrations.

Currently we only connect to the database in development, as we don't
yet have the database setup for the staging/production environments.

Release Notes:

- N/A

crates/collab/migrations_llm/20240806182921_create_providers_and_models.sql

@@ -0,0 +1,16 @@
+create table if not exists providers (
+    id serial primary key,
+    name text not null
+);
+
+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
+);
+
+create unique index uix_models_on_provider_id_name on models (provider_id, name);
+create index ix_models_on_provider_id on models (provider_id);
+create index ix_models_on_name on models (name);