Support unit test outside

[wenhaocs]

Clients may want to write unit tests with nebula-go client. They will mock the Execute interface and return a mock ResultSet. But there is no public function to generate a mock result set. The members inside result set is also private.

We should (1) either create a mock ResultSet and mock Record. (Users are using mockery, //go:generate mockery --name=Client --outpkg=mocknebula --output=./mocknebula)
(2) or expose a public method to construct ResultSet and Record.