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/src/lib.rs

@@ -166,6 +166,7 @@ pub struct Config {
     pub openai_api_key: Option<Arc<str>>,
     pub google_ai_api_key: Option<Arc<str>>,
     pub anthropic_api_key: Option<Arc<str>>,
+    pub anthropic_staff_api_key: Option<Arc<str>>,
     pub qwen2_7b_api_key: Option<Arc<str>>,
     pub qwen2_7b_api_url: Option<Arc<str>>,
     pub zed_client_checksum_seed: Option<String>,
@@ -216,6 +217,7 @@ impl Config {
             openai_api_key: None,
             google_ai_api_key: None,
             anthropic_api_key: None,
+            anthropic_staff_api_key: None,
             clickhouse_url: None,
             clickhouse_user: None,
             clickhouse_password: None,