PPL - query to return all PPL commands

**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
}
...
]
