i3bar: Support customizations in workspace buttons protocol

## I'm submitting a…

[ ] Bug

[x] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior

The i3bar workspace button protocol only supports the standard key-values of the `GET_WORKSPACES` command

## Desired Behavior

We can enhance the protocol with configuration that is already supported by i3bar (examples copied from https://github.com/i3/i3/issues/3818):
* `"min_width":32` (related to #3711)
* `"separator":true` — personally I find it is useful to visually separate groups of workspaces.
* `"color":[...]` — could be used to distinguish fake empty workspaces.

## Impact

[ ] This feature requires new configuration and/or commands

## Environment
Output of `i3 --moreversion 2>&-`:

i3 version: 4.22