`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.
