Current commit introduces distributed SQL in picodata (we use
'sbroad' library). Now it is possible to execute cluster-wide
SQL with 'pico.sql([[query]], {params})' lua method.

Current limitations:
1. DDL is still not implemented in picodata, so space sharding
   is mocked by '_pico_space' space (we'll remove it later).
2. We still have duplicating symbols of the Tarantool module in
   sbroad and in picodata rlibs.
3. We need to reuse piocdata's sharding functions and handle the
   case when different spaces in the query have different sharding
   functions.

Co-authored-by: Georgy Moshkin <gmoshkin@picodata.io>