search_path support

[Artjoms Iskovs]

Support SET search_path either in SQL as a temporary context-altering statement:

-- table bla.table_1 exists
SET search_path = 'bla'; SELECT * FROM table_1;   -- when this is a multi-statement single query, e.g single HTTP / gRPC request

or as some metadata header in Arrow gRPC, whichever is simplest.