ra-rpc: populate Unix peer creds for UDS endpoints

[kvinwang]

This PR exposes Unix-domain peer credentials to RPC handlers and wires the existing Unix-socket callers to preserve that information.

Changes

• add UnixPeerCred { pid, uid, gid } to ra-rpc
• extend RemoteEndpoint::Unix to carry peer: Option<UnixPeerCred>
• add UnixPeerCredListener, a small Rocket listener wrapper that reads peer credentials from accepted Unix streams via UnixStream::peer_cred()
• use that listener in guest-agent's internal Unix-socket servers so handlers receive peer creds in CallContext.remote_endpoint
• add unit tests for endpoint mapping and peer-cred propagation

Validation

• cargo check -p ra-rpc -p dstack-guest-agent
• cargo test -p ra-rpc
• cargo clippy -p ra-rpc -p dstack-guest-agent -- -D warnings

Notes

• This is a source-breaking API change for downstream code that pattern-matches on RemoteEndpoint::Unix(...); callers need to switch to RemoteEndpoint::Unix { path, peer }.
• When peer credentials are unavailable, peer remains None.