ratelimit: redesign

Description of how this works in `docs/internal/RATELIMITS.ms`.

Notable implementation changes:
- KEEP_SEND_PAUSE/KEEP_SEND_HOLD and KEEP_RECV_PAUSE/KEEP_RECV_HOLD
  no longer exist. Pausing is down via blocked the new rlimits.
- KEEP_SEND_TIMED no longer exists. Pausing "100-continue" transfers
  is done in the new `Curl_http_perform_pollset()` method.
- HTTP/2 rate limiting implemented via window updates. When
  transfer initiaiting connection has a ratelimit, adjust the
  initial window size
- HTTP/3 ngtcp2 rate limitin implemnented via ack updates
- HTTP/3 quiche does not seem to support this via its API
- the default progress-meter has been improved for accuracy
  in "current speed" results.

pytest speed tests have been improved.

Closes #19384

tests/data/Makefile.am

@@ -280,7 +280,7 @@ test3032 test3033 test3034 test3035 \
 test3100 test3101 test3102 test3103 test3104 test3105 \
 \
 test3200 test3201 test3202 test3203 test3204 test3205 test3206 test3207 test3208 \
-test3209 test3210 test3211 test3212 test3213 test3214 test3215 \
+test3209 test3210 test3211 test3212 test3213 test3214 test3215 test3216 \
 test4000 test4001
 
 EXTRA_DIST = $(TESTCASES) DISABLED data-xml1

tests/data/test3216

@@ -0,0 +1,19 @@
+<testcase>
+<info>
+<keywords>
+unittest
+ratelimit
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<features>
+unittest
+</features>
+<name>
+ratelimit unit tests
+</name>
+</client>
+</testcase>