[FLINK-21466][table] Add SQL Client default mode value embedded

[zck573693104]

1.Add SQL Client default mode value embedded.
2.Modify sqlClient md,support default mode value embedded.
3.Test sql client default mode embedded.only left a test which uses embedded to verify it still work.

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[fsk119]

Thanks for your contribution and sorry for the late response. I left one minor comments.

[fsk119]

rename model to mode

[wuchong]

I have appended a commit to remove useless embedded and also added a test for SqlClient.

Would be great if @fsk119 can have a look again.

[fsk119]

Thanks Jark's help. The added test is amazing.

LGTM.

[wuchong]

Merging...