Restore multipart query parameter support

[ruslandoga]

Context

master used to support multipart: true for Ch.query/4. The rewrite removed it for now while simplifying the request path around NimblePool, eager Ch.query, default RowBinaryWithNamesAndTypes decoding, and explicit request/response compression.

Old behavior worth preserving:

• multipart: true was an option on queries.
• Query params were encoded with the same ClickHouse escaped parameter encoding, but sent as multipart parts named param_<name> / param_$<index> instead of URL query params.
• The SQL statement was sent as a multipart query part.
• x-clickhouse-format was still set on the request, defaulting to RowBinaryWithNamesAndTypes unless overridden.
• URL query string still carried ClickHouse settings.

Proposed restore scope

Restore multipart only as a transport encoding for query text + query params:

Ch.query(pool, "SELECT {value:String}", %{ "value" => large_value }, multipart: true)

Implementation shape:

1. Keep public API as Ch.query(pool, statement, params, opts).
2. When multipart: true, build a multipart body containing:
   • one query part with the SQL statement;
   • one part per encoded query param, named param_<name>.
3. Keep settings in the URL query string, not in the multipart body.
4. Set content-type: multipart/form-data; boundary=....
5. Preserve the response behavior from the rewrite:
   • default decoded RowBinaryWithNamesAndTypes returns %Ch.Result{names, rows, headers, data};
   • overridden formats return raw successful bodies;
   • ClickHouse errors return %Ch.Error{}.

Inserts

Keep inserts non-multipart for now.

For inserts, the current raw body model is clearer and works well with compression:

payload =
  :zstd.compress([
    "INSERT INTO table FORMAT RowBinaryWithNamesAndTypes\n",
    Ch.RowBinary.encode_names_and_types(names, types),
    Ch.RowBinary.encode_rows(rows, types)
  ])

Ch.query!(pool, payload, %{}, headers: [{"content-encoding", "zstd"}])

That keeps content-encoding scoped to the entire body ClickHouse parses after decompression. Multipart insert support can be considered separately if a concrete use case appears.

Compression interaction

If multipart: true is combined with content-encoding, compression should apply to the fully-built multipart body, not to individual parts. This is different from compressed inserts, where the caller can pass a precompressed complete insert body directly.

Tests to restore/add

• named params sent through multipart round-trip correctly;
• large string params that would be unsuitable for URL query strings;
• settings still go into the URL and work with multipart;
• custom response format with multipart still returns raw body;
• content-encoding: zstd or gzip compresses the full multipart body, if we choose to support that combination;
• inserts continue to use the raw body path and are not affected by multipart unless intentionally supported later.