error: introduce error_payload

It is a dictionary-like struct which stores keys with binary data
values. The values are supposed to be arbitrary MessagePack blobs:
number, string, bool, UUID, array, map, anything.

The payload is an array inside instead of a hash table because
number of keys will be <= 3 in all cases. And even when it will be
public, it is very unlikely it will be bigger.

Object of error_payload in a future patch will be stored in struct
error and will allow to extend it dynamically with more members.

This in turn is needed to extend ER_READONLY error with more
details about why it happened.

Part of #5568
Part of #4907