collab: Use a separate Anthropic API key for Zed staff (#16128)

This PR makes it so Zed staff can use a separate Anthropic API key for
the LLM service.

We also added an `is_staff` column to the `usages` table so that we can
exclude staff usage from the "active users" metrics that influence the
rate limits.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

crates/collab/migrations_llm/20240812184444_add_is_staff_to_usages.sql

@@ -0,0 +1 @@
+alter table usages add column is_staff boolean not null default false;