Motivation:

When a chunked HTTP message contains a trailer field with a folded value
(obs-fold continuation line starting with SP or HT), readTrailingHeaders
threw UnsupportedOperationException. This happened because the folding
logic called List.set() on the list returned by
trailingHeaders().getAll(), which is an AbstractList implementation that
does not override set().

Modifications:

Changed readTrailingHeaders in HttpObjectDecoder to accumulate folded
continuation lines into the instance-level `value` field, mirroring the
pattern already used by readHeaders. The previous header is now flushed
via trailingHeaders().add() only once its complete value is assembled,
eliminating the need to mutate the list returned by getAll(). Forbidden
trailer fields (Content-Length, Transfer-Encoding, Trailer) are filtered
at flush time, consistent with the previous behaviour.

Added tests to HttpRequestDecoderTest and HttpResponseDecoderTest
covering:
- A single trailer field
- A folded trailer field with multiple SP and HT continuation lines,
followed by a non-folded trailer to verify isolation between fields
- Forbidden trailer fields interleaved with a valid one, with a
forbidden field placed last to exercise the post-loop flush path

Result:

Chunked HTTP messages with folded trailer values are now decoded
correctly instead of throwing UnsupportedOperationException.

(cherry picked from commit 9a85f9f)