PPL - query to return all PPL commands

[mengweieric]

Is your feature request related to a problem? Please describe.
Auto-complete needs to get all available commands.

Describe the solution you'd like
PPL query to get all commands

Preferred return format

[
...
{
name: search,
expl: 'search' // brief explanation for this command
... // any other useful information related to this command
}
...
]

[dai-chen]

There is SHOW FUNCTIONS statement in SQL world. Probably we can provide similar show function/commands.