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

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