Add DistSQL to show or set Proxy props.

[RaigorJiang]

Hi community,

ShardingSphere-Proxy has many props, but they are only displayed in the configuration file, and currently users cannot query them through SQL.
We plan to add a DistSQL that allows users to query or modify some configuration parameters of Proxy, such as the value of sql-show.

Before, we already had the RAL syntax:

• show variable xxx;
• set variable xxx;

Now, for Proxy's props, we can use these syntaxes:

• show variable sql-show;
• set variable sql-show = true;

But how to show all Propxy props?

Or add a new keyword for props:

• show props sql-show;
• set props sql-show = true;
• show props all; # Show all props

I think the second one is better, but I hope to get some suggestions. Any comments are welcome, thank you.

In addition:
The property names in props are all separated by a dash (-), which does not match our SQL variable specification, but modifying the variable name has a greater impact on users, so it is specifically explained here.

[tristaZero]

Looks cool, I prefer the first proposed pattern, as it's suitable to join the RAL group.

[RaigorJiang]

Thanks @tristaZero . I will start to develop it.