fix(gateway): bound every peer HTTPS exchange by the sync timeout

[kvinwang]

HttpsClient (hyper-util legacy client) has no connect or request timeout. SyncManager bounds sync rounds with sync.timeout, but fetch_peers_from_bootnode calls the client directly, both in ProxyInner::new (before the proxy binds its listeners) and in the retry loop. A bootnode that completes the TLS handshake and never answers hangs the gateway at startup.

HttpsClientConfig gains a timeout, set from sync.timeout (30s default), applied to each request including the body read.

Verification: new test a_peer_that_never_answers_does_not_hang_the_client (before the fix: Elapsed(()) after 10s). cargo test -p dstack-gateway 309 passed; fmt and clippy clean.