Add a default-on, size-aware foreground PUT admission policy so large or
unknown-size PutObject requests are backpressured before body ingest and
erasure/RPC fan-out. Preserve the explicit strict gate semantics, including
limit=0 as an opt-out, and keep small PUTs on the legacy fast path.
Closesrustfs/backlog#2038
Co-authored-by: heihutu <[email protected]>