aws_credentials: fetch_credentials_from_vsock is async but does blocking VSOCK I/O

[AnthonyRonning]

Context\nA PR review noted that is but performs blocking VSOCK operations (, , response read + JSON parse). This can block a Tokio runtime worker thread.\n\n already uses for similar VSOCK operations.\n\n## Why it matters\nBlocking I/O inside an can stall the Tokio runtime worker thread and delay unrelated async work (timers, request handling, other tasks).\n\n## Suggested fix\nRefactor to run the blocking VSOCK connect/write/read/parse inside (or make it sync and call it via ), keeping existing socket timeouts and bounded reads consistent with ping.\n\n## Location\n- : \n