http/2, http/3: decouple stream state from easy handle

- add `Curl_hash_offt` as hashmap between a `curl_off_t` and
  an object. Use this in h2+h3 connection filters to associate
  `data->id` with the internal stream state.
- changed implementations of all affected connection filters
- removed `h2_ctx*` and `h3_ctx*` from `struct HTTP` and thus
  the easy handle
- solves the problem of attaching "foreign protocol" easy handles
  during connection shutdown

Test 1616 verifies the new hash functions.

Closes #13204

tests/data/Makefile.inc

@@ -205,7 +205,7 @@ test1590 test1591 test1592 test1593 test1594 test1595 test1596 test1597 \
 test1598 \
 test1600 test1601 test1602 test1603 test1604 test1605 test1606 test1607 \
 test1608 test1609 test1610 test1611 test1612 test1613 test1614 test1615 \
-\
+test1616 \
 test1620 test1621 \
 \
 test1630 test1631 test1632 test1633 test1634 test1635 \

tests/data/test1616

@@ -0,0 +1,22 @@
+<testcase>
+<info>
+<keywords>
+unittest
+hash
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+</features>
+<name>
+Internal hash_offt create/add/destroy testing, exercising clean functions
+</name>
+</client>
+</testcase>