Rename assistant_settings to agent_settings (#31513)

This PR renames the `assistant_settings` crate to `agent_settings`, as
well a number of constructs within it.

Release Notes:

- N/A

crates/eval/Cargo.toml

@@ -19,8 +19,8 @@ path = "src/explorer.rs"
 
 [dependencies]
 agent.workspace = true
+agent_settings.workspace = true
 anyhow.workspace = true
-assistant_settings.workspace = true
 assistant_tool.workspace = true
 assistant_tools.workspace = true
 async-trait.workspace = true