v4 folder for transition period

[asmyasnikov]

For example...

• interface ydb.Connection have method Table() and returns table/table.Client interface
• type ydb.Driver impolements ydb.Connection interface and returns table/table.Client
• suggestion:
  • ydb.Driver will have method Table(), which will return v4/table/table.Client struct instead implementation of table/table.Client interface (internal/table/table.Client struct)
  • v4 folder can helps owr customers for switch own code into next major release (v4)

This idea helps