feat(daemon): keep tokens in the secret store

[noctuum]

benshi_daemon::secrets keeps the token of a backend: Secrets reads, writes and removes it by a Key. Memory keeps tokens in the process, and SecretService, on Linux, keeps them in the store that answers the Secret Service API on the session bus.

• A token is an item of the default collection with the attributes application = benshi and backend = <backend>, passed in a plain session.
• A call made with Asking::Never does not ask the store to unlock and is Error::Locked. A call made with Asking::Within waits that long for the person the store asks.
• Every call is given up on after the deadline the store is made with. DEADLINE is 5 s, and PROMPT_WAIT, the wait for a person, is 2 minutes.
• The tests run against a Secret Service of their own over a socket pair, and against a session bus that is not there or does not answer.
• zbus 5.19.0 is added for Linux, with p2p in the dev-dependencies.

The ignored tests/secret_store_live.rs was run once against ksecretd of kwallet 6.31.0 with the wallet unlocked: the write, read, replace and removal passed and no window was shown. A locked wallet was not tested.

cargo test --workspace passes, 980 tests.