Add `IPROTO_UNKNOWN` command code for overriding the unknown request
handler.

Change request type variable types related to IPROTO to `uint32_t`.

Add request handler hash table to transaction thread and request handler
set to IPROTO threads for storing overridden request handlers: TX thread
notifies IPROTO threads about overridden request handlers using IPROTO
configuration message.

If a given request handler is overridden, the IPROTO thread does not
preprocess it and sends the package immediately over a dedicated route. If
later it is necessary to fallback to the system handler, the message
decoding and dispatching is done in the TX thread.

Add new `box.iproto.override` method to Lua and `box_iproto_override` to C
API, which allow setting IPROTO request handler callbacks.

Closes #7901

@TarantoolBot document
Title: Document overriding IPROTO request handlers feature
For the API description and usage examples, see:
* [design document](https://www.notion.so/tarantool/box-iproto-override-44935a6ac7e04fb5a2c81ca713ed1bce#0f84694523214c0e9bf2f3d75cccace4);
* tarantool/tarantool#7901.