lua/msgpack: enable encoding errors as msgpack extension

This patch adds a new msgpack.cfg: encode_error_as_ext. Setting it makes
msgpack and msgpackffi modules encode errors as the MP_ERROR msgpack
extension. If the flag is unset, msgpack.encode behavior depends on
encode_load_metatables, encode_use_tostring, and encode_invalid_as_nil
options, see luaL_convertfield(), while msgpackffi.encode() will always
encode errors as strings. The latter needs to be fixed, but it's out of
the scope of this work and tracked separately, see #4499.

The new option is enabled by default.

Interaction with box.session.settings.error_marshaling_enabled: errors
are encoded as the MP_ERROR msgpack extension when returned via IPROTO
iff both error_marshaling_enabled and encode_error_as_ext are set.

Closes #6433

@TarantoolBot document
Title: Document msgpack.cfg.encode_error_as_ext

The new option determines how error objects (see box.error.new) are
encoded in the msgpack format:
 - If it's set, errors are encoded as the MP_ERROR msgpack extension.
   This is the default behavior.
 - If it's unset, the encoded format depends on other msgpack
   configuration options (encode_load_metatables, encode_use_tostring,
   encode_invalid_as_nil). With the otherwise default configuration,
   they are encoded as strings (see error.message).

Functions affected by the default configuration (msgpack.cfg):
 - msgpack and msgpackffi modules
 - Storing errors in tuples and spaces (box.tuple.new)
 - Returning errors from IPROTO CALL/EVAL