Flip189 flinksql client features

[snuyanzin]

What is currently missing?

There is a FLIP189[1] which was accepted however not yet reviewed.
This FLIP among others suggests this set of features

• Line continuation pattern
• Syntax highlighter (alt-h to change highlight style)
• Command completion
• Line numbers (alt-shift-n to toggle line numbers)
  demo for all points mentioned above: https://asciinema.org/a/446247?speed=3.0
• Prompt handler (right and left prompts) demo: https://asciinema.org/a/458191?speed=4.0

How could this be improved?

I implemented and ported this feature to Flink 1.13.x branch.
So need to update dockerized flink-sql-client from https://github.com/snuyanzin/flink/releases/tag/flip189_1

[1] https://cwiki.apache.org/confluence/display/FLINK/FLIP-189%3A+SQL+Client+Usability+Improvements