[pgAdmin4][Patch]: SQL Auto completion feature for Query Tool

I have implemented the sql auto complete feature for Query Tool in pgAdmin4. To achieve this I have analyzed the code of "pgcli (http://pgcli.com/)" and also used some part of the code "(https://github.com/dbcli/pgcli )" with our own modification. 

"pgcli" buffers all the data(databases, tables, functions etc...) at once while connecting to the database, but we can't do that, so when user press "Ctrl+Space" in the Query Tool we will fetch the data and display the suggestions accordingly.

Attached is the patch file. Please review it and let me know the review comments (if any). 

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