The new IPROTO protocol feature IPROTO_FEATURE_ERROR_EXTENSION enables
encoding errors returned by CALL/EVAL commands as the MP_ERROR MsgPack
extension. Note, the MP_ERROR extension can still be disabled globally
by setting msgpack.cfg.encode_error_as_ext to false. If an IPROTO client
doesn't set the feature bit, errors will be encoded as generic cdata
objects (converted to strings by default).

Needed for #6428

@TarantoolBot document
Title: Document IPROTO_FEATURE_ERROR_EXTENSION

A new feature bit for the IPROTO_ID command was added:

```
IPROTO_FEATURE_ERROR_EXTENSION = 2
```

The protocol version was incremented - now it equals 2.

If a network client sets this bit, errors returned by CALL/EVAL will be
encoded as the MP_ERROR MsgPack extension (unless disabled globally by
msgpack.cfg.encode_error_as_ext). If the bit is unset, errors will be
encoded according to the serialization rules used for generic cdata
objects (converted to strings by default).

The built-in net.box connector sets this feature bit.

The server sets this feature bit if it supports the MP_ERROR MsgPack
extension so a net.box client can explicitly request the feature upon
connecting to a server:

```lua
net.box.connect(uri, {required_protocol_features = {'error_extension'}})
```