assistant: Add tool registry (#17331)

This PR adds a tool registry to hold tools that can be called by the
Assistant.

Currently we just have a `now` tool for retrieving the current datetime.

This is all behind the `assistant-tool-use` feature flag which currently
needs to be explicitly opted-in to in order for the LLM to see the
tools.

Release Notes:

- N/A

crates/assistant/Cargo.toml

@@ -25,6 +25,7 @@ anthropic = { workspace = true, features = ["schemars"] }
 anyhow.workspace = true
 assets.workspace = true
 assistant_slash_command.workspace = true
+assistant_tool.workspace = true
 async-watch.workspace = true
 cargo_toml.workspace = true
 chrono.workspace = true