per #5471, verifiers stop depending on the queue client. the module is
store-free and needs only thiserror, so swarm-agent-verify gains one
dependency and loses its swarm-queue-client edge. consumers import it
from swarm_agent_verify; swarm-logs and swarm-metrics, whose only use of
swarm-queue-client was this module, drop that dependency, as does
swarm-logs-auth.
`CredentialSource`, `Store` and `verify` move from swarm-nats-auth's
`agent_token` module into a small library crate, so a second verifier
can make the same check: the secret in a `swarm-agent.<agent>.<secret>`
token equals the one stored at `swarm/agents/<agent>/queue`, read under
the verifier's own certificate.
`verify` now takes its timeout as an argument; swarm-nats-auth passes
`INTROSPECTION_TIMEOUT` as before. No behaviour change. The store-denial
tests moved with the code; swarm-nats-auth keeps the ones that go
through its policy and its bound.
Part of #4459.
