feat: optional gzip compression for ingest

[acoshift]

Summary

Adds opt-in gzip compression for the ingest path (review item #5 from the earlier performance pass). NDJSON is highly compressible, so for log-shaped workloads this trades a little CPU for a large reduction in wire bytes and latency.

• New SetGzip(true) enables it. Off by default — existing behavior is unchanged.
• When enabled, each batch is gzip-compressed and sent with Content-Encoding: gzip. The Quickwit endpoint must accept gzip-encoded ingest requests.
• The gzip writer is per-worker and reused across flushes via Reset, so the compressor isn't reallocated per batch — matching how the JSON encoder/buffer are already reused.
• Compression happens after the empty-body check and before request construction, so the 413 auto-reduce/chunking path compresses each chunk independently. Sending via bytes.Reader still lets net/http set Content-Length (compressed size).

Test plan

• New TestIngest_GzipCompressesBody: server requires Content-Encoding: gzip, decompresses, and asserts every record is delivered in order. (If the header were missing the server 400s, so the test genuinely exercises the encoding path.)
• All existing tests implicitly cover the default-off path (they read the raw body without decompression).
• go vet ./... clean
• go test ./... and go test -race ./... pass

Notes

• Gzip always compresses when enabled; very small payloads can expand slightly, but this is opt-in for callers who know their batch profile.

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