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

lib/hash.c

@@ -368,3 +368,25 @@ void Curl_hash_print(struct Curl_hash *h,
   fprintf(stderr, "\n");
 }
 #endif
+
+void Curl_hash_offt_init(struct Curl_hash *h,
+                         unsigned int slots,
+                         Curl_hash_dtor dtor)
+{
+  Curl_hash_init(h, slots, Curl_hash_str, Curl_str_key_compare, dtor);
+}
+
+void *Curl_hash_offt_set(struct Curl_hash *h, curl_off_t id, void *elem)
+{
+  return Curl_hash_add(h, &id, sizeof(id), elem);
+}
+
+int Curl_hash_offt_remove(struct Curl_hash *h, curl_off_t id)
+{
+  return Curl_hash_delete(h, &id, sizeof(id));
+}
+
+void *Curl_hash_offt_get(struct Curl_hash *h, curl_off_t id)
+{
+  return Curl_hash_pick(h, &id, sizeof(id));
+}