Add maxTrailerCount attribute for HTTP/1.1 chunked request trailers

[HwangRock]

HTTP/2 has a maxTrailerCount attribute (default 100) that limits the number of trailing headers in a request, but HTTP/1.1 has no equivalent. On HTTP/1.1 the chunked trailers are bounded only by maxTrailerSize (bytes), so the number of trailer fields is effectively unlimited within that byte budget. Regular HTTP/1.1 headers already have a count limit through maxHeaderCount, and the commit message of 2aa5f6b noted that maxHeaderCount and maxTrailerCount could be added to HTTP/1.1 "at some point". maxHeaderCount was added since; maxTrailerCount was not.

This adds maxTrailerCount to the HTTP/1.1 connector, mirroring the HTTP/2 attribute. The value is threaded from AbstractHttp11Protocol through Http11Processor into ChunkedInputFilter, which counts the trailer fields as they are parsed and fails the request with a 400 once the limit is exceeded. It reuses the same parse loop that already enforces maxTrailerSize, so the enforcement and error path match the existing trailer handling.

The default is 100, matching maxHeaderCount and the HTTP/2 equivalent. This does change behaviour: a request with more than 100 trailers that previously passed (within the maxTrailerSize byte limit) is now rejected. A value less than 0 disables the limit. I went with 100 for consistency, but I'm happy to default it to -1 if leaving the existing behaviour unchanged is preferred.

Tests in TestChunkedInputFilter cover below/at/above the limit, the disabled (-1) case, the zero case, and a check that trailers under the limit are still parsed. A changelog entry is included.