ws: pause/unpause write handling

Websocket frames need to be written individualy, so that applications
can access the frame meta data correctly. This worked until the write
function triggered a pause. Remaining frames accumulated in the "out"
writer's buffer and on unpaused were written in one chunk.

ws decode writer will now stop writing frames when the client writer
is paused. To handle the writing of buffered raw data after an unpause,
client writers have gotten a new "flush" method.

Add pytest test_20_12 with a new client to check handling of pauses
and websocket frames.

Reported-by: Hendrik Hübner
Fixes #22273
Closes #22283

lib/content_encoding.c

@@ -299,6 +299,7 @@ static const struct Curl_cwtype deflate_encoding = {
   NULL,
   deflate_do_init,
   deflate_do_write,
+  Curl_cwriter_def_flush,
   deflate_do_close,
   sizeof(struct zlib_writer)
 };
@@ -360,6 +361,7 @@ static const struct Curl_cwtype gzip_encoding = {
   "x-gzip",
   gzip_do_init,
   gzip_do_write,
+  Curl_cwriter_def_flush,
   gzip_do_close,
   sizeof(struct zlib_writer)
 };
@@ -493,6 +495,7 @@ static const struct Curl_cwtype brotli_encoding = {
   NULL,
   brotli_do_init,
   brotli_do_write,
+  Curl_cwriter_def_flush,
   brotli_do_close,
   sizeof(struct brotli_writer)
 };
@@ -606,6 +609,7 @@ static const struct Curl_cwtype zstd_encoding = {
   NULL,
   zstd_do_init,
   zstd_do_write,
+  Curl_cwriter_def_flush,
   zstd_do_close,
   sizeof(struct zstd_writer)
 };
@@ -617,6 +621,7 @@ static const struct Curl_cwtype identity_encoding = {
   "none",
   Curl_cwriter_def_init,
   Curl_cwriter_def_write,
+  Curl_cwriter_def_flush,
   Curl_cwriter_def_close,
   sizeof(struct Curl_cwriter)
 };
@@ -704,6 +709,7 @@ static const struct Curl_cwtype error_writer = {
   NULL,
   error_do_init,
   error_do_write,
+  Curl_cwriter_def_flush,
   error_do_close,
   sizeof(struct Curl_cwriter)
 };