client writer: handle pause before deocding

Adds a "cw-pause" client writer in the PROTOCOL phase that buffers
output when the client paused the transfer. This prevents content
decoding from blowing the buffer in the "cw-out" writer.

Added test_02_35 that downloads 2 100MB gzip bombs in parallel and
pauses after 1MB of decoded 0's.

This is a solution to issue #16280, with some limitations:
- cw-out still needs buffering of its own, since it can be paused
  "in the middle" of a write that started with some KB of gzipped
  zeros and exploded into several MB of calls to cw-out.
- cw-pause will then start buffering on its own *after* the write
  that caused the pause. cw-pause has no buffer limits, but the
  data it buffers is still content-encoded.
  Protocols like http/1.1 stop receiving, h2/h3 have window sizes,
  so the cw-pause buffer should not grow out of control, at least
  for these protocols.
- the current limit on cw-out's buffer is ~75MB (for whatever
  historical reason). A potential content-encoding that blows 16KB
  (the common h2 chunk size) into > 75MB would still blow the buffer,
  making the transfer fail. A gzip of 0's makes 16KB into ~16MB, so
  that still works.

A better solution would be to allow CURLE_AGAIN handling in the client
writer chain and make all content encoders handle that. This would stop
explosion of encoding on a pause right away. But this is a large change
of the deocoder operations.

Reported-by: lf- on github
Fixes #16280
Closes #16296

lib/sendf.h

@@ -182,6 +182,8 @@ CURLcode Curl_cwriter_write(struct Curl_easy *data,
  */
 bool Curl_cwriter_is_paused(struct Curl_easy *data);
 
+bool Curl_cwriter_is_content_decoding(struct Curl_easy *data);
+
 /**
  * Unpause client writer and flush any buffered date to the client.
  */