agent: Extract usage information from response headers (#29002)

This PR updates the Agent to extract the usage information from the
response headers, if they are present.

For now we just log the information, but we'll be using this soon to
populate some UI.

Release Notes:

- N/A

crates/agent/Cargo.toml

@@ -90,6 +90,7 @@ uuid.workspace = true
 workspace-hack.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true
+zed_llm_client.workspace = true
 
 [dev-dependencies]
 buffer_diff = { workspace = true, features = ["test-support"] }